Ok - this is a big if - but if it's just a single weber - the main 'gain' from webers (well aside from the fact they are a pretty decent design in general) is where you have one carb throat per cylinder/intake port. If you don't have that (which you won't with a single weber) then the gains over twin SUs is much smaller. THe weber (if you optimise both setups) will still outpower the twin SUs, but not by very much, and i'd bet money nobody would be able to tell without lifting the bonnet, just by driving.

the big if, however - is as Matto says - the weber is easier to setup (since it's a matter of altering jet, idle jet, emulsion etc (though I'd bet you'd get the emulsion right in 1-2 goes tops), With SUs, you need to buy 2 needles per 'experiment' till you get it right. If it were decades ago, more than a few workshops would have enough needles on hand that they could dial it in and just bill you for the time and final needle choice, but now, good luck, and the needles are expensive all things considered.

That said, try giving SUmidel (I think there is a website too) a call. They are (or were) a firm in NSW if I recall, who specialised in the SU carbs and might just have enough experience to get you right in the ballpark as far as needle selection, possibly enough to be acceptable (if not perfect) for a lot of apps.