Tuning in the ST world has moved on alot recently there are now a few running 400bhp+ and i believe Jamsports RS actually has a modified ST block running 500bhp.
For what your looking for you need an intercooler, decat and remap. Dreamsciences DSCI will run very well with them mods and you should see atleast 270bhp and over 300 torques. Second hand this lot will set you back £500-£600.
If you want to go for more power then you can get a CAIS and turbo downpipe then get a map to suit. The most common would be Dreamsciece Mod X but alot of people including myself have moved to Collins stage 2 which blows Mod X out the water. 320-330 bhp and nearly 400 torques. Very smooth delivery it just pulls and pulls all the way to the redline.
Oh and dont forget the obligatory Eibach lowering springs.