What would you do next?

[tulowd]

You are missing the most obvious mod that most people also don't do.

Tires are the only contact point with mother earth, and any car with over 350 hp needs sticky rubber in order to maximize traction, grip, response and agility.

Unless you are running Cup2 Michelins, you are leaving a pile of performance on the table without sticky tires like a NT01, or similar disposable sticky DOT track tire. Torsen diff follows closely behind.

I went from some "performance tires" to Nitto NT555s to Toyo R1R's to big Nitto NT01s.
Car felt like it gained 100rwhp; ask the Gallardo, F430 and supercharged M3 that I handed it to in one day 2 summers ago shortly after Nittoing.

This is with an old Fox convertible, naturally aspirated with (only) 444 rwhp/380 tq; your car is on another level already, only you can't run with Timberlines or wooden shoes strapped on, lol

Maximum Grip/Friction is everything, the rest is a waste of time and money until you are pushing the limits of the performance envelope your car is currently capable of.
Ask JT - The Mustang Show.....he knows

[KRS]

I agree, tires are always very important, see the second paragraph on post #15. Tires are on my list but not for this year, unless I come across a good deal on a set of rear 11/12 SVTPP wheels to have widened.