Speaker performance in a not ideal listen room/area

If my listening room (basically family room) with distance from left wall to left speaker is about 20ft while the right side is about 6ft to the right wall and about 50ft to the back wall from speakers.

Is it true that regardless how much acoustic treatment or how much you change to it or how good or expensive the speakers, they will not perform to its fullest in this type of irregular room? Basically a $1000 and $50000 speaker will be about the same due to the room?

I am trying to upgrade the speakers but then wonder if I will ever get the same feel I get from the dealer's audition room, even using the same amp and cables. Last thing I want is to upgrade to expensive speakers and then cannot listen to the best of it!

Thanks for advice.