Aftermarket exhaust systems are excellent in increasing engine power and turbo response. Gains as high as 15hp on an otherwise stock engine care often realized. Exhausts typically run $450 on up.
Many companies offer exhaust systems for the MR2 Turbo of which can be broken down into those that retain the secondary catalytic converter and those that do not. You should note that you might run into emissions problems if you install one of the latter types.
Manufacturers included, but are not limited to, APEX’i, HKS, Greddy, and Thermal Research & Development. Numerous custom setups are possible as well.
In the MR2 community, the APEX’i GT-spec and Greddy Power Extreme are popular, although the latter is difficult to obtain.