When we brewed our last pumpkin beer, I roasted several pie pumpkins and some butternut squash. I had over 2 quarts of mashed squash, and I could taste it in the wert before and after spicing it. I do think using pie pumpkins as opposed to jack-o-lantern pumpkins (these are worthless for flavor) and roasting the pumpkin makes a difference. I'm not convinced that just using pumpkin pie spice in a beer will have the same roundness/depth of flavor as actually using pumpkin. It may taste like pumpkin pie, but I just wonder how two made side by side, one with and one without pumpkin would compare in a taste-off.
Russian Imperial Stout
Imperial Pumpkin Ale