kud13: Thespian Feast is basically one of the late-game locations from the full game. Same enemies for the most part. It ends with beating the same boss, but there's no way out of the boss room (that I could find, at least- and you don't get a new Rift once you beat him to teleport out).
The only new piece of content is that you can grab a boss-based weapon in the Thespian Feast before you face him- and in the actual game you need to kill the boss to be able to craft this.
I honestly haven't found all secrets in Thespain Feast, and I discovered some extra areas in the same map in Hellpoint, so I can't say for sure if there's any major "spoilers" Thespian Feast has. But for the most part, they are pretty similar.
I did do the Thespian Feast area first in my Hellpoint playthrough (because I found the key to that area in the secret area of the Embassy), but i'm pretty sure you're NOT supposed to go there early, because you're insanely underleveled for it.
I was able to get through it, (because I knew the area from Thespian Feast), but that left me over-levelled for most of the game, so it leads to some serious sequence-breaking.
Ah ok, thanks a lot for explaining! So it's basically a slightly adapted section from the late game and no significant miss to skip and directly jumping into full game.