Most cost effective power mod is usually a filter on our cars - but the gains are relatively limited on the later cars with its larger air box and dual air intakes - just make sure that these are directed into nice, cold air.
Exhausts broadly make about 5bhp/5 lb.ft for the best ones (the good old Daytona was best, with the Milltek Supersport just behind).
4-2-1 manifolds broadly don't make much extra power but can make oddles of torque lower down the rev range where you most need it, and the best ones can really transform the way the car drives (the Powerspeed one on my car is absolutely brilliant). Most manifolds are optimised for the MPi head and cams; the VVC's requirements are different and this is likely the reason why the "one size fits all" solution fails to work so convincingly on VVC heads!
For MEMS3 users, we saw at our RR day that Mark's Z&F re-tune netted a good, reproducible 5 bhp gain with much improved driveability. I don't think I've ever heard a bad word about this conversion!
Beyond these changes, you start spending ever more money for ever less gain, but you could consider hotter cams - piper's BP270H might be a good choice, and the 633 a more powerful choice. Also think throttle bodies early on: Jenvey offer a bolt-on upgrade. MEMS3 could be re-programmed to run these, and would be worth discussing with Mark, or consider getting an Emerald. Thereafter talk to my old friend, Dave Andrews, regarding a head port and bigger valves - but bear in mind that an exhaust manifold suitable for a lesser engine will no longer be optimal for an engine in a higher state of tune...
Also consider whether the pistons and rods are man enough for a higher rev limit required to get the most from the higher state of tune. But the good news is that you could potentially be looking at 200-220bhp...
How far do you really want to go?
