I know these cars quite well as it happens.

You will need the block mod before you go any further, costs around £700.

Then the standard ST pistons and rods will limit you to around the 350-360bhp mark, so worth considering if you want to switch to either RS internals, (good for 400bhp in an ST), or forge it, depending on your budget, at the same time as the block mod.

You can push a Revo Stage 2 programme to around the 350bhp mark on a hybrid turbo and big injectors, but it's not strictly ideal. Devil Developments can do it though and the map is around £420.

You would see more gain (in terms of power and drivability) moving to an aftermarket standalone ECU. But you're looking around £2600 for Syvecs, fully fitted and mapped.

In your current spec, with decent plugs you should see around 330bhp on Revo Stage 2, with a block mod.