The best mod to do is to buy the kaizen tc4 conversion. lol. Nice stuff. I'm buying one next friday. I'll post up how it turns out.
Running my vta tc4 the best upgrade I noticed. Or at least made my times go down the most was a bearing kit. I would recommend Boca bearings. Wait for a coupon And buy the best ones you can afford.
The next best thing is the factory team hinge pin arm mounts. The ones that have the O X and what not on them.That made for about a .5 second decrease. Just remember run the 3 degrees X rear unless the track is somewhat straight And you have hela traction then get the 2.5.
Another good upgrade was the factory team shocks with the VCS2 upgraded bladders in them. That gave me another easy .3 seconds. Plus makes it way more reliable and you don't need to worry about leaks.
The cheapest upgrade with the best gain has to be to change the stock rod ends for the open tc7 ones. They will fit on the ft rods and they stay tight for waywayway longer.