The beauty of a top of the line car is that it doesn't really need any upgrades to perform well. The only upgrades would be to fine tune the car such as springs, diffs, sway-bars, shock pistons and other items of that nature. The other area you may upgrade is for durability by getting the alloy pulleys and stronger belts. Other than that there really isn't any reason to upgrade.

I found that the car out of the box, with the appropriate tires and shock oil, is about 90% dialed just by setting it up as indicated in the manual. The last 10% is more or less track dependent.

If you are running stock, 17.5 then the 1.89 version would give you the timing boost you want.