porter - I have just started running the S3 for TTR. Right now the parts that I know are suspect out of the box are the shock towers (have a tendency to bend) and possibly the steering slider. If you got a newer kit it should have come with a one piece center diff brace to use instead of the 3 piece carbon/aluminum one. You definitely want to use the one piece part. Racers Edge was (still is?) having a sale on the TTR shock towers so that would be a good way to go and you get the option camber link holes on the rear shock tower too.
The newest thing to do is to modify the steering slider by cutting the ears off of it and using longer tie rods to mount under the bellcranks like a Mayhem or TTR S2 buggy. This eliminates the need for an aftermarket steering slider and takes away all of the bump steer. I built my kit stock for now but once I get the longer tie-rods I will be doing this mod.
A lot of guys are taking the Xray or Mugen shock pistons and drilling them to 1.5mm or 1/16"(about 1.6mm) and using them instead of the stock pistons. Then they use the Xray light blue springs although the spring rates are very close to the red TTR springs.
Other mods that are considered useful: red upper front hinge pin holder is thicker and protects the plastic bushings better in crashes, lightweight pillow balls are hardcoated and probably improve suspension smoothness in addition to being lighter than stock, red rear suspension holders should be stronger than stock plastic as is the FR0 plate for the front. Do not use the red aluminum parts for the very front lower suspension plate (it is called FF-1, FF+1, or FF0). The stock part is stronger and easier to make adjustments on.
Finally, most people have gone to the flat chassis conversion which makes the car longer and gives it more flex. TTR is supposed to release standard and long wheelbase conversions and Titan tech makes the original long wheelbase setup. The Titan tech is 6mm thick and machined out while the TTR is 4mm and not machined out.
To get on Sgrid you need to go to the free conferences from the www.sgrid.com