Yeah personally I got a set of front GT calipers off a guy in my local club for around $100, got some new brake lines from O'Reilly's and some new drilled rotors and pads for the front and rear to match and it made a massive difference. The rear brakes are the same as the GT so it's still balanced, and you don't have to do anything extra besides swap in the parts. The eco brake booster etc all plays nice with the GT calipers.
Other than that I can personally recommend Koni STR.T orange struts for a decently smooth ride. I had put steeda lowering springs on my stock struts and it was rough, but once I got the konis it was really smooth for the price without being too track focused compared to some of the more expensive ones. Other people ride in my car and are amazed at how smooth it is whilst being so low. Pedders coilovers seem to be pretty smooth in my friend's GT, but I don't have that kind of money!
I also did a front strut bar, lower 2pt steeda k-brace, front and rear BMR sway bars and endlinks, steeda bushing anti-deflection kit(the metal things that bolt around the subframe bushings, I had to remove one set to put the BMR bracing in place), steeda subframe alignment inserts, BMR heavy jacking rails and rear BMR subframe reinforcement/bracing. I never changed out the stock bushings or put in the cheaper bushing reinforcement inserts, I always figured those would detract from the ride comfort in relation to the performance benefits. All of that together put on over a few years really tightened the car up whilst still being smooth enough for my wife to put her makeup on the road.
Hope that helps and if you have any questions about any of that just ask, I put it all on myself and only horribly destroyed one endlink in the process!
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