Well if you wanna get nit-picky, the original poster never said anything about water, they called it a "gas tank", and gasoline would weigh about 330 pounds, a drum weighs about 40 pounds, so I'll go as high as 380 total if fuel. If it was water, then your 500 estimate sounds reasonable...
If a Kirby 25 couldn't stay ahead of a Shark 24 in 20+ knots, it had nothing to do with a 55 gallon drum on the high side in the cabin.
First, a 55 gallon drum of gas only weighs ~300-350 pounds; second, a 55 gallon drum wouldn't fit inside a Shark without cutting the deck off first; and third...