Do more expensive bib shorts make a difference when sat on a turbo trainer for hours on end?
The big difference between rollers and the outside is that your position is much more static inside. If you are good enough on rollers to stand every once in a while that might help. ( I'm not ). Maybe just take a 5 minute break every hour. 

If your shorts/saddle are comfortable outside for 2+ hrs, I doubt expensive shorts will really make much of a difference. However, when it comes to saddle comfort everything works for somebody and nothing works for everybody. 

I'd try the expensive Assos cream to start with, it's only a $30 experiment vs a $300 one.