The engine can handle up to 800bhp with stock internals, although it is asking for trouble.
The first things you need to do is, get a full system exhaust, either a performance air filter in the stock airbox or a induction kit, Fuel Cut Defender and Boost Controller. Also look into getting a BOV as it will definitely help with Compressor Surge when running higher boost.
One of the quietest exhaust is the Fujitsubo Aristo Exhaust system which is available from Japan, apparently the most "popular" exhaust as it's a very high flow design which isn't very loud whatsoever compared to stock.
Air Filter I'd go for a APEXi Power Intake, and make a custom heatsheild as opposed to using the stock airbox. I'm planning out my induction box as we speak using the stock front "nozzle", should be done in the next two - three weeks. I'll post up pictures if you want.
BOV, personally I've gone with the HKS Super SQV due to it's design, and well it does give off a VERY sweet sound lol.
Boost controller I'd go with the Blitz Dual Solenoid R version (can't remember model number off the top of my head), as it's supposed to simple to use and enables a sharper boost curve than any others Boost Controllers out there.
After doing all of that, I'd look into getting the Autobox rebuilt with uprated parts. If I'm not mistaken Whifbitz do a Boostlogic autobox upgrade. This uprated gearbox is supposed to be able to handle 1000+ rwhp easily. Although the most expensive upgrade you will have to do, it's the most worthwhile as you'll have no doubt eliminated the weakest link in the whole car.
Another thing I'd look into is uprated brakes, even running uprated discs and pads will be enough, or if you're in need of some more stopping power, you can always upgrade to the Supra TT disc & caliper, or if you've got deep pockets look into the Endless USA kit for the Supra which I'm told will fit the Aristo. Just make sure you've got enough clearance for the Endless kit as the discs are MASSIVE!
If you haven't already got a Turbo timer, GET ONE! Even if you have to sit inside the car or wait for the car to turn off, it's worth it as a good turbo timer will calculate how long the engine needs to stay on.
That's all I can think of right now off the top of my head. Goodluck!