Raceproved have previously done refurbs with roller bearings and double lip seals to prevent leakage issues, might be worth a call to them to see if they're still offering the service.
Have you tried swapping the butterflies around between the individual throttle inlets? If you have an early car with alloy butterflies you can sometimes bend these gently to reduce the airflow .. i've done this with some success in the past. Be careful though!
