Depends on what kind of money you have to spend. FMIC, HKS Downpipe, get rid of the cat put in a straight pipe, 3"exhaust, fuel computer(greddy emange), injectors(440's out of JDM 2jz or 550's out of USDM 2jz with resister pack), walboro fuel pump, FPR not sure who makes the best, intake(HKS makes the best IMO unless you get a custom made one), boost controller. With all of this and some tuning at 1bar you could very easily make 370rwhp. I have seen it with my own eyes and plan on doing this, I am most of the way there. With the y-pipe ou could get more I am sure which is why I want one I just wish I could find one.
you should peep the writeup on the 1jz upgrade path.
it may or not be possible for you to do a y-pipe. they are pretty labor intensive to install/build one. its one of those optional things if you wanna do it. but, just do intake, exhaust, boost controller and safc
you can get 300rwhp easily.
changing the y-pipe will get you a little more. but youll need either a FCD/BCC to get around the 1.0bar boost limit or just bump the fuel system and scale back w/ the safc.