Standard UK spec Supra has 326BHP. So for your 400BHP you only need to make 74BHP. Around 50BHP is allegedly achieveable by fitting a decent exhaust system (Blitz, HKS) and a full de-cat (it has 2 cat's, get rid of both of them). This will also transform the car mainly through it's quicker spool up of the turbo's and nicer sound. Budget around £800 for this. That leaves you with 24BHP to gain - easily achieveable with a small increase in boost pressure, say 13psi instead of 11. You can do it the 'proper' more expensive way with an electronic boost controller by Blitz, HKS, Apexi etc. or you can do it the simple mechanical way - this way is cheaper but you won't be abe to adjust the boost in car. May be an idea to fit a decent boost guage as well to keep an eye on things.
*** DISCLAIMER *** I'm a n00b too looking for a Supra TT and as such don't have any real experience!