As said the T4 is very unusual, a T34 is normal the route for that spec and is a good turbo, but there again it depends on what you personally want from the car, you simply don't need the bigger turbo in your current setup as you won't need to make the boost that it is potentially capable of, the RS500 had a T4 but only for homologation purposes, this is why they are laggy, in race trim they had eight injectors to supply enough fuel for the boost produced.

Personally I would be looking to change the turbo as this will make the car far more driveable!

Martin