What is the 1G in at the moment?
Easy and first ones are normally get a good, decent exhaust system. Get your Intake sorted out. Slap on an Intercooler that's not overly big. Just big enough to do what you need it to do.
Then get an EBC and you can wind the boost up a bit. I'm not sure on what boost cut is with the 1G, I have forgotten after the Supra.
If you get problems with boost cut just get an FCD and you're away. Watch out for the turbos though as they are not bulletproof. Wind it out too much and they'll pop like corn in a hot pan.
You could go with a bigger single turbo on the motor but there's allot that needs changing for that sort of thing. And yes you could run it from your stock ECU....but what's the point? If you are spending all of the money to convert to single turbo then why not fork out a lil extra and get the whole thing tuned properly with an aftermarket ECU?
That way you'll get the best power out of it.