ANSWER:
There are many gains to be realized by upgrading the clutch in your Mustang. However, a clutch change on that application involves more than the clutch itself, and you will want to do everything right the first time because it is labor intensive.
There are several considerations we address when making a recommendation, and the end-user should rank or prioritize these before choosing a high performance clutch. The first consideration is torque capacity, or how much power you need it to hold. That is most often the first priority, as you have stated that your current unit is not holding @ 400 rwhp and you have plans for 550rwhp. The proper formula by which to rate a clutch is ft-lbs torque, not horsepower. Based on the nature of your modifications, you should be able to determine your torque level and most of the time it will be within 15% above or below your horsepower. As an example, we will assume your setup includes a turbo kit and will make at least 525 wheel torque. It is recommended that a 10-15% capacity buffer be added to your goal to be safe and allow for underrated modifications and/or greed for more power. That buffer can also help maximize clutch life. So a clutch with a 600 ft lb capacity would be a good choice. Based on our ratings, any stage from 2+ past 5, as well as our street/strip twin disc, will support your power. Our ratings take into consideration that sticky tires may be used, as well.
The second consideration is drivability, or how you want the clutch to feel at engagement and pedal requirement. Pedal requirement is not a concern on the Mustang with a SPEC clutch, as the hydraulic system lends to ease of use and we now include the LPR (Light Pedal Requirement) option on all 2005 and newer Mustang and GT500 kits. The friendly feel at engagement will be important since you drive the car on the street and likely incur stop-and-go situations. You would probably want a fairly smooth engagement, as well. The choices for a street-friendly engagement would narrow the field to a sprung hub (dampened) disc and a material configuration that is somewhat forgiving. Based on the characteristics of our stages, the field would be narrowed to the 2+,3 , 3+ and twin disc. The stage 2+ feels very much like stock, the 3+ is very street-friendly and the stage 3, though very streetable for a puck clutch, has a noticeably more aggressive engagement.
The third consideration is life expectancy. It is very hard for a clutch manufacturer to rate or guarantee life expectancy simply because there are many after-sale factors that determine the wear cycle. Clutch life will be maximized as long as you stay within the clutch capacity (choose with the buffer as explained above) and follow special instructions that accompany a high performance clutch, as well as industry standard installation practices like replacing old bolts, refreshing worn hydraulics, surfacing or replacing the flywheel, etc.. Given instructions are properly followed during and after installation, a high performance clutch not only will last a lot longer than an original equipment clutch in a modified car, but also outlive a an original equipment clutch in an unmodified car. The SPEC stage 2+ , 3+ and twin disc offer excellent life cycles, and stage 3 would offer a good cycle.
The fourth consideration is weight. Some street and track driven cars can benefit from a clutch that is lighter in weight. The benefits can be in throttle response and horsepower/torque. The lightweight clutches will not produce benefits or changes in drivability as drastic as a lightweight flywheel, neither are they as costly. A lighter clutch alone will not produce any substantial downside and is a great way to complement your round of modifications with a part that can produce gains without adding stress to any other part of the car. Engines internals will benefit from a lighter rotating assembly. SPEC offers and aluminum pressure plate option for all Mustangs @ $199. This option reduces clutch mass by as much as 40% and creates a rebuild-ability feature for the pressure plate.
The last consideration is price. Value will be based on how you prioritize the above considerations. Basically, a clutch is like taxes. Spend sufficiently now or your will be penalized in the near future with additional parts and labor costs!

Now that we have narrowed the clutch choices, it is time to address other clutch related components. The factory flywheel, which is the component that most contributes to the performance, life and drivability of a clutch, is a cast iron unit that is only serviceable to a certain thickness. We recommend taking only .020 from a cast flywheel before replacement, for obvious performance and safety reasons. You can purchase a performance flywheel not much more than a cast factory replacement. SPEC offers two billet flywheel options (steel and aluminum). Both are infinitely rebuild-able, manufactured to an industry best .001 machine tolerance and can enhance the performance of your Mustang. I can make a recommendation on which would be best for your car based on the nature of your modifications, where you drive or race, and tire compound choices. Normally we recommend steel for naturally aspirated cars that drag on slicks and large-turbo cars, with which keeping boost up in between gears is important. We recommend aluminum for all other combinations, including road racing, autoX, street and drag racing with a lot of mods and proper gearing.
Secondly, determine the feasibility of retaining or replacing the 3V concentric slave/release bearing assembly. We recommend changing them on every other clutch change or every 50K miles. They can be included in our kits for $99.
So, to finally answer your question, I would only be able to give a range of recommendations based on the information given. The choices would be stage 2+, 3, or 3+ clutch kit, billet steel or billet aluminum flywheel, all of which would be shipped as a precision balanced assembly from SPEC with all necessary hardware for installation included. (bolts, release bearing, guide tool). I look forward to talking to you about your priorities to finalize which clutch stage and flywheel material will complete the best possible clutch solution for your 3V!

David Norton- SPEC Clutches and Flywheels
800-828-4379