I have brewed this same recipe in the past, and it is a good beer if done correctly. The review that stated their beer was watered down with no pumpkin or spice flavor likely brewed it incorrectly. They likely added too much top-off water following cool down, a common newbie error.
There is not a SINGLE Midwest Supplies brew kit recipe that isn't well balanced, well calculated, and proven. Any problems with any of their kits is going to be brewer error 99% of the time.
There's no reason to add extra pumpkin to the brew. The pumpkin doesn't contribute much of the pumpkin taste, it's mostly the spices.
I'd leave the beer alone if I were you, but if you must, boil 1 tbs of pumkin pie spice in about a cup of water and GENTLY swirl that into the beer.