factory rods are pretty strong.. people put new pistons in, underbucket shims, balance things up and then most start looking at new super-duper turbo kits.
ATS Racing and KO Racing are two companies both staffed by hard-core MR2 people (hi Ken!) that will point you in the right direction.
First, start with the important things. making sure every hose, wire, fitting, etc is in good working order. Can't make power on a crapped out old powerplant.
Headwork is nice, it depends really on how much you have to spend.. I believe on person is over 700hp on a 3SGTE engine right now.. so the possibilities are there.. but you know how things go. "Speed costs money, how fast do you want to go?"