Re: 9" vs 8" vs 8" dual booster
The 8" dual will provide slightly more assist then the 11" single, so it definitely provides much more than a 9". The "formula" is the square area of the diaphragms multiplied by the vac inches, so to speak, assuming the same vac it becomes a matter of calculating the surface area of the diaphragms for comparisons sake. A bigger booster is capable of more assist at a given vac, A 9" dual works very well, 11" dual(truck size) even better, however just doesn't look or fit as well as the 8" dual. Most that I know run the 8" dual with good success.
Back to your vac issue, 7-8"'s ain't going to cut it with ANY booster.