AFAIK the ST is a somewhat restricted engine, so first steps are get more air in and more fumes out, there is a 40mm bottle neck on the exhaust and the standard airbox is restrictive...
150BHP standard and with K&N57i, full exhaust, cams, inlet and remap it will be 200 (approx)
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Black Mountune ST185
Mountune enclosure and K&N57i
Milltek exhaust system including 4-1 manifold
Piper cams & valve springs
Remap