i know some supra mods can be spendy, but you can get some pretty decent gains with a few low-budget mods.
1. better intake/air filter .. turbo engines need good airflow to make power. a better intake will help increase the amount of flow. the apexi filter/adapter unit should net you a few horses.. here's the one i have ..
clicky clicky .. make sure you get the adapter as well, not sure if that particular listing includes it (most likely not)
2. better intercooler/piping .. again with teh better airflow, the stock piping is a little restrictive. you can find front mount intercooler sets on ebay for a coupla hundred bucks, and most come with some piping as well. another nice thing about a good front mount is that it will lower the air temp which also picks you up some power. i don't have one yet, but everything i've looked at points to gains of 20-30 hp.. which is not too shabby for a couple bills
3. better downpipe .. again with the airflow (sensing a theme here?) the stock downpipe is restrictive because of the pre-cats. replace that with a 3" straightpipe and you can pick up quite a few more horses. you can either have a local shop whip you up one for cheap (straight pipe with flanges shouldn't cost more than $100-150 or so) or you can order a pre-made one for around $250
4. better exhaust .. with the downpipe, a good 3" mandrel bent exhaust should net you a good 30hp. you can either buy a ready made piece or have it custom bent at a local shop. mine had a 3" system on it when i bought it, but i did have one done for my crx.. complete 3" cat-back (including piping, muffler, resonator, tip,mounts and labor) set me back a whopping $280.
picking up extra horses on a stock turbo'd engine is pretty simple. you can take it even farther with bigger turbos and engine re-enforcing.. but you should definately get used to the car's power and handling before you get into that.
then again, the supras stock form might be well enough for him.. 230hp and 246tq is nothing to scoff at.