you will need a real expert to alter the mapping in your ecu. and with atmospheric aspirated engines the power increase will only be a couple of % without getting trouble in some areas such as idling.
sc or turbocharching is the only realtively simple way otherwise flowing, camshafts and valves are the only way to increase power.
It's illegal in the Netherlands, bu how about popping in alarger engine if you're that desparate?
I never have the impression being short of power with my VX.
And be carefull cause youre tranny will blow if you overdo it.