fwiw the externally referenced powervalve was a necessity on drawthrough turbo setups with holleys - at least those I played with. They were an interesting scenario - the powervalve would open when the acclerator was floored, and it'd 'work' for a moment, then as the turbo spooled up and the onset/increase in boost occured, the vac between carb and turbo would increase and pull the darn thing shut again :)

With respect to the acc pump dribble, there are alternate acc pump squirter 'block' designs that can assist there simply by creating a 'dead' or low airflow area just outside the squirter discharge point. The phenomenon also occurs when a 2 barrel is put onto an engine that is way to large for it (which is the case for certain race categories in the US to restrict output, not that I know which categories they specifically are!!) - they setup a bleed on the back of the metering block in the acc pump circuit to help prevent it. That means (although it's not cheap) you can actually source an off the shelf 2 barrel that is prepped to prevent that, or alternately you could modify an existing metering block (though to be honest it's not actually something I've had need to do).

I can absolutely see where you are coming from with respect to the fact that superchargers are instant flow/boost and the rise of a piston on an SU type carb isn't (and shouldn't be, as it will actually dull throttle response, as it leans out momentarily if it opens too fast) instantaneous. No argument there, but I would add that at part throttle, once you are actually operating on the main metering circuit of a fixed venturi carb, it's venturi cross section and booster size/design that determine signal strength and mixture delivery/consistency, and having enough carb to supply the peak rpm/throttle needs will still result in (arguably) less than ideal signal strength at part throttle. Without a doubt the throttle response and output will be good for full power/competition stuff, but it can (and of course it depends on just how big the carb is relative to the application) be less ideal for part throttle.

Having said all that, it's pretty obvious that there's plenty of drawthrough (and blowthrough) setups out there that use either of the types of carbs being discussed, and the result is great either way provided the carb is set up to suit the application!