"Never flew the 5 - been told that the pressurisation bleed reduces the power to about that of the 3"
Not so. The Mk3 had a viper 101 series engine rated at 1650lbs (IIRC) and the Mk 5 had a Viper 201 series engine rated at 2450lbs (again IIRC). The Mk5 was a rocket ship compared with the athsmatic Mk3. Perhaps you are thinking of the Mk4 which also had the 201 and no pressurisation. I often heard the Mk4 was a better performer than the Mk5, but chaps who had flown both said it was impossible to detect any difference in performance.