Wes Ingram used to offer manifolds with larger butterflies, but this has been gone from his website for some years. Jim Steck enlarged the bores on my SPICA throttle bodies some years ago, but retained the stock butterflies. I installed these along with some other modifications so was not able to identify any improvement.
Way back in the day people used to "knife edge" butterflies in SU carburetors and file down the attachment screws. I obtained a spare set of throttle bodies to do this, but the project has become dormant.
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1976 Spider (Dedicated Autocrosser, "SPICA, No Carbs")
1991 Spider Veloce (Retirement cruiser)
Scuderia Non Originale