You need vacuum advance on any road car, unless its a full race engine in which case it will have a totally different distributor curve to suit. Regards to extra grunt from a weber, setup correctly there will be little difference between a pair of SUs of 1 1/2" (yours in the picture by the way are 1 1/4s and would restrict th bhp on a well sorted 1330) and a weber 45 except at the very top end of the rev range. Check Vizards book on the graph and its only a few bhp were talking about.
So back to your question it depends on what the spec is of your engine, if its built to 1275s spec and about the same BHP (75) then the SU 1 1/4s will be ok and fitting the weber will do little, if on the other hand your motor is built and your expecting 100bhp then the weber would not be restrictive like the little SUs
The vac take off is quite important as if its sited in the inlet manifold then its subject to a higher vacuum then if its sited slightly behind the butterfly as per SUs and in Bungles Dellortos above. They started fitting later cars with the vacuum takeoff in the manifold for emission reasons as at a given rev it produced more vacuum advance which was a attempt to clean up emmission readings. The problem with it being in the manifold is induction pulsing which upsets idle and the sportier the engine the worse it becomes. Your problem is the Weber was sold as a racing carburettor and is not drilled for a ported vacuum take off, which is only there to aid economy.
Not really but it will "see" vacuum faster. I've seen cars with little brass orifice nipples to smooth out the way vacuum is transferred
