I spent nearly £2500 on bolt on parts for my ST170 and that does not include fitting, off the top of my head

£850 Clutch and fly wheel
£250 Blue Finn
£400 Dreamscience
£150 K&N Typoon
£100 CFM quickshift
£90 Exhaust Cam Vernier retarded
£350 Mongoose Cat back exhaust
£120 Underdrive Pulleys
Ported Throttle Body


And from all the above the best i got was 189bhp.
The engines are already tuned, so not very tunable, your filter may have even lost you power. Unless you want to spend £3000+ on a supercharger kit or £4000 upwards on a turbo charger conversion i really would not bother.