I did roast the canned pumpkin. The first thing I noticed was that there was a lot of lost volume. Obviously the canned pumpkin has a lot of water content that gets lost during the roast. After roasting I added the pumpkin to the mash. I was sort of surprised by how easily it dissolved into the water.
That led to something I didn't consider. The pumpkin would add to the overall volume. As a result I ended up with a lot more than two gallons so I had to reboil about a quarter of a gallon down to a very thick almost caramel-like consistency and added it into the fermenters. Since I was using two 5l jugs I had limited space and as it is I still have more than two gallons.
The pumpkin also provided an...interesting mash experience. I mashed BIAB-style so I should have had no problems with stuck mashes or sparges due to the pumpkin. No, actually the pumpkin gummed up the grains so much I couldn't get the nylon bag to drain. I had to cajole it with a spatula and spend a lot of time breaking up the clumps of grain to get it to drain. Eventually after several dips in the sparge water it thinned out enough to drain on its own.I saved the grain to make bread. I hope that it carries a little pumpkin flavor into the bread.
The wort pre-boil tasted like wort with squash in it. Not really a pumpkin flavor as much as a generic pumpkin flavor. If you have never tasted pumpkin before it is pie you may not be aware that the spices in the pie really enhance the pumpkin flavor and turn it into something much less squash-like. As the boil concentrated flavors it began to get more of a pumpkin aroma. When I added the spices at 5 minutes before the end of the boil it was like all the pieces came together and it got a really good pumpkin pie aroma. The taste post-boil was strongly pumpkin pie.
I think the pumpkin is a necessity. Otherwise, you just have a beer with spices in it. The spices might remind people of pumpkin pie but I think a more discerning palate would disagree that it tastes like pumpkin. It seems to me that a beer with spices and no pumpkin is just a winter warmer. There's nothing wrong with it, it's just not the same thing.
I'll see how the flavor is once fermentation is done and the beer has had a chance to settle and mellow. I pitched the beer on the cake of a one gallon 60 shilling ale so it had a good chunk of yeast to work through it. It appears fermentation is close to finishing already, thanks to the low ABV and massive yeast count. I'll probably let it sit 3-4 weeks in the fermenter before I taste it to see if it needs more spice. If it does I'll let it go another week or two and try again. Then 3-4 weeks in the bottle. That should make the beer ready to drink mid or late October (maybe early November) to bridge the gap between Oktoberfest and winter beer.
Next year I would like to remake the beer. Although I would probably look at using fresh spices (e.g. cinnamon stick over cinnamon powder that is of an unknown age) over dried as much as possible. I might also play with adding vanilla, as that seems to be a fairly popular ingredient this year. I also would definitely like to try using fresh pumpkin instead of canned. Not that I am disappointed with the canned pumpkin but I think fresh pumpkin would add a fresher taste and hopefully is easier to deal with.
Anyway, I hope some of this is helpful and best of luck to everybody brewing those pumpkin beers.