I don't know for sure, but that seems like too much. I've been researching pumpkin ales, and most call for a 10 lb fresh pumpkin (less after you gut it), or around 2 - 4 lbs. of canned pumpkin. I could be wrong because they might be trying to offset the rich flavor of a porter. I made a pumpkin porter last year. It was a mr. beer recipe, and only called for 1 can. that was of course a 2.5 gallon batch, but I could definitely taste the pumpkin. I wish there was a little more, but I'm sure that 2 cans would have done it. It's up to you on the amount of pumpkin flavor that you're looking for, but 7.5 lbs does seem like a bit much.