I hope this is of interest.
Here is a run down summary of the mods with what was used, costs and gains.
Stage 1:
Toyosport silencer and exhaust manifolds. I managed to buy them on eBay for £130 and £110. They are the generic stock pictures you see on most listings. Fitting was done in 2 stages and cost approximately £100 and £180.
Peak bhp gain +9bhp
Stage 2:
74mm 996 throttle body and plenum from Porschapart, £100 and £30; 12" 83 mm ID aluminium pipe and 2 x 83mm internal diameter 4 ply silicone hoses from autosiliconehoses.com, £12 and 2 x £18; 80 - 100mm jubilee clips approx 6 needed, £1 each. Replace new paper air filter with a cotton K&N panel filter. Cost to fit = DIY with forum member Patt
Peak bhp gain +6bhp
Stage 3:
Custom remap at Motorsport Developments in Blackpool. I was referred here by another forum user Mark Shead. Cost £350.
Peak bhp gain + 6bhp
You can see from the graph the gains in power and torque for each set of mods. After stage 1 it really is chasing ever diminishing returns.
It may be feasible to swap stage 1 and 2 around yet still get the same result.
If you never drive your car hard or rev it out then these mods are a bit of a waste of time. Of course the flipside is the better sounds you get from stage 1 and 2.
With each of the mods they start to take effect around 3000 - 3500 rpm and hold the increased power and torque from 4500 rpm all the way through to the red line.