The stumble is more than likely a carburation issue and not due to vacuum.
It sounds like you are right on with the master/booster adjustment. Leave it as-is.
Cheap pads usually equal soft pads. Soft pads usually stop better cold than high performance pads. Don't get me wrong, there are some cheap pads out there that don't stop for a damn.
Lets keep working with what you have before you go and spend any more money.