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Agree with the above - to a point. The all FP kit on my GT350 was objectively inferior to replacing the rear springs with BMR. There was a good discussion on this on one of the forums - but basically Ford went to a softer spring in the rear to compliment the kit's thicker sway bar. Supposedly decided to prioritize managing roll more than out right grip with the kit (or something to that nature). The result was the rear squatting too much and causing tons of mid corner understeer once you got on throttle. The BMR springs reduced the amount of squat and made substantial improvements to the mid corner grip for me.
Now - that's for the GT350. I only bring it up as I think it's important to take blanket statements with a grain of salt. I generally agree with the sentiment above. Mixing and matching on your own without having a ton of knowledge on car setup and tuning is likely going to lead to worse overall performance. So going with something like the FP kit is smarter than mix n' match, IMHO.
A real set of coilovers would be even better, tho