A couple of idle comments on this unusual (to me) post. How the muffs fit surely affects performance, as does water pressure. With a properly fitting set and adequate pressure the flow should be better since there is no lift requirement....pressure preloads the impeller. Once the engine starts it uses some of the flow (sucks on it) so the excess will decrease.
In a drum, the water level is probably well above the water pump so it doesn't have to perform any "lifting" function. That may not be a good test of the integrity of your water pump's impeller. If you are talking about a 55 gallon drum that must have been interesting to get that engine in that drum and the reason for my initial comment. I never had any problem with dads 10 hp Scott Attwater as a kid, but a 115..... Other thing is running in a drum, your exhaust heats the water so you need to keep an eye on that.
