The TC05-10A was used on Mitsubishi Raiders, Pajeros and L300 vans and there appears to be both waste gate and non-waste gate versions (google image came up with TC05's with waste gates so who knows). An engine does not need exhaust 'back pressure' but an engine manufacturer will try to find a compromise with exhaust size as they want a balance between low down torque and top end exhaust gas volume. I would get a better flowing turbo with a waste gate and go up 1/4" to 1/2" max for exhaust diameter. This should get the engine onto boost earlier and be able to keep up with exhaust gas flow on open throttle instead of strangling it. The main problem with the 4D55 is the turbo is too small for the engine and it runs out of steam quickly. I think you will need to figure out how to improve the fuel delivery curve as well. Better turbo + decent exhaust + fuel will vastly improve drivability.