just on another note, can you modify SU's to apply mechanical advance ?
In the old days I had a 13b- RX4 (with standard downdraught carb) that I had the secondaries modified to mechanical advance instead of vacuum, it was a 1/2 hour job and made a great difference.
Ive got series 6 Su's (less than 6 months old) and they have run faultlessly apart for some minor timing issues.