Turbo stage 2.5: 3 inch exhaust, injectors and where the limits are

An owner was planning a "2.5 stage" on his Turbo: a bigger intercooler, direct intake, balancer shaft removed, cooler spark plugs, an oil cooler, and a 3 inch decat, totally straight exhaust like the picture below. His questions and the replies are summarised here. Treat the figures as owners' estimates, not tests.

Noise and legality

• One owner thinks a straight decat pipe is a bad idea and will sound horrendous. He doesn't know of a technical back-pressure reason on a Turbo, but says it wouldn't meet UK track noise limits and would fail an MOT, so the car wouldn't be street legal. The poster, who was in Italy, said the MOT-equivalent rules there weren't a problem for him.
• Another owner fitted one to a customer's car a couple of years earlier and says it didn't sound too bad.
• An owner with experience says most people who fit a 3 inch exhaust use a Tullet, which is not loud and allows track use. See Exhaust backbox choices and noise.

Injectors and power limits (one owner's view)

• Bigger injectors help going forward but aren't essential at stage 2. Another owner suggested VXR injectors with the ECU adjusted to the new injector map. The poster bought injectors anyway.
• Maximum power on standard injectors is around 250 bhp, give or take.
• Maximum power on a 2.5 inch exhaust is around 300 bhp. A 3 inch exhaust will allow a lot more.
• Air filters don't really make more power.
• Going past stage 2, the biggest issue is heat, and a bigger intercooler won't solve it because of where it sits. The two options he gave are methanol or water injection (which lowers inlet temperatures), or a charge cooler.
• Stage 4 needs an 80 mm MAF, plus a clutch, a VXR turbo and injectors, and water injection or a charge cooler.
• He says water injection halved his inlet temperatures and allowed more timing in the map, creating more power.