dump valve is also not required...
But I'd put money on most people wanting on it for the noise.
The less power you run the more you can save (as above) - standard vts clutch can cope with a boosted 130hp engine (as you'll see around 130ft/lb also). Same with injectors - just use vts injectors, £20, sorted.
Go from 130hp to 200hp though and you'll start to see significant cost increases as other bits cant cope (clutch, exhaust, injectors, rad, ecu etc etc). Then to 300hp (a real 300...) and it's a much larger jump. Then 400 is a monumental jump, and above that is well, you get the jist...
