Stage 1. Improves Performance (Bhp and Torque from 10-20%) depending on Car Turbo, Engine and Gear Box. It makes the car smoother and improves milage.

Stage2. Boosts Power(BHP and Torque)15-25% by deleting Exhaust Gas Recirculation, Diesel Particulate Filter,Catalytic Convertor. ( Adds pops and bangs for all petrol cars and fire spits for twin turbo cars)

Addition of Down Pipe or Full Exhaust System.

Stage2+. Is accessible for most cars, it boosts power from 25-30%. 

It requires upgrading :- 

Upgraded Intercooler, Performance Air intake, Full Exhaust system or Down PIpe.