I think your summary, on one or three carbs, is pretty accurate for the vintage car. I was underwhelmed by the first vintage Six I drove with three carbs (I was accustomed to the single Stromberg), expecting more noticeable performance improvement, but the second car I drove had definitely sprightlier acceleration (maybe the first was simply not tuned well). Top speed on this second car was maybe five/six mph better but, remember these models are restricted by the gearbox and the r/axle worm and wheel ratio. That's why AC, late 20's, offered various packages - different camshaft, high comp. pistons, three carbs, different ratio gears and final drive. For example, the Monte Carlo car of Bruce was a 16/66 but still had one updraught carb, the extra h.p. coming from these engine mods. Also, for consideration, the standard vintage cars have gears designed to get into top quickly, so from 3mph to 60+ mph in top was possible (a selling point which was advertised to good effect). The 'three carbs' car may accelerate a little better and have a marginally higher top speed but there is a point where you have to say, with a standard VINTAGE vehicle, 'is this worth the effort'? The 'Sports/Montlhery' model got around this by having all the mods here, including a better diff ratio thus top speed was increased by up 15mph or so, and the car was capable of prolonged high speed driving.
A number of vintage AC cars that have been changed to three carbs do use SU's but another variable is which ones? I think AC, for example, used six variants of SU's over the years. Tuning, plus the carb types, making a difference to end result. For example, the needles on your car you say are marked 'L'. Your carbs therefore are not the same as AC offered from the vintage period. 'L' was the standard needle from AC's of the mid-30's, when also 'weak' and 'rich' alternatives were offered. Its size is .090in. There were carb sets before your type where the needles were different and another part number...I think 'WX', if I recall correctly? For further information you could try Burlen Fuel Services in Salisbury who are the experts on SU carburettors (and they do offer many needle types and sizes to suit your carburettors).