Keeping a Beetle running cool is a very good idea but I wouldn't be concentrating on the exhaust to accomplish this goal. Better to look at the air-cooled cooling system. Make sure that
all of the engine "tin" is in place. Make sure that the engine compartment rubber seals are all in place such that the engine compartment is absolutely sealed tight with the exception of the louvres in either the engine compartment lid or just above the engine compartment lid. The whole goal here is cooling air in from up above and discharged down below all accomplished by that big fan inside the cooling shroud. I have seen Bugs where you could look into the engine compartment and see road underneath - NO WAY. Keep that stock oil cooler in place and in first class shape. Make sure that your thermostat is opening the path for the air flow or , and this is open to debate, remove the thermostat and wire the louvres in the cooling shroud permanently open. These few steps will go a long way toward a cool running engine.
I'd rather have a partial bottle in front of me than a partial frontal lobotomy.