I've never seen a leaking master cylinder cause a booster to fail. I've seen a booster fail & suck fluid past a worn out master cylinder, though. However, if the back side of the master cylinder is the slightest bit damp you might as well replace it now while it's loose. They sell a booster/master cylinder combo also. It saves you about $10 that way, opposed to buying them separately. Remember to bench bleed.

Then again, you don't want to spend a ton of money on the brakes if the engine is burned up. You might want to clamp off the vacuum supply to the booster & test drive. Don't forget, your brakes won't stop very well w/o power assist. Be careful. That way you'll know if the vacuum leak from the booster is or is not the culpret for the overheating.