My 83 Spider has a leaking brake booster, which I intend to replace as funds become available. My question is, I know Alfas are notorious for hypersensitivity to vacuum leaks, can the brake booster vacuum leak affect idle or running quality?
Yes. Intake vacuum is the 'power' source for the brake booster.
Also check the one-way valve screwed into the intake plenum (where the hose attaches). Remove it and test by blowing through it. You should easily blow through is towards the intake and not be able to blow through it the other way. It can be cleaned with a few sprits of carb cleaner.
By leaking brake booster, do you mean a booster unit that's not airtight, or brake fluid leaking into the booster? If it's the latter, then the engine will run just fine except when it's sucking brake fluid into cylinder 4... then it'll run kinda rough, due to low octane level of the brake fluid... and turn your exhaust into a smoke bomb.