Re the difference between the ATR and Q400, I don't find much comfort difference as pax. The 400 feels like a much bigger aeroplane than the ATR for some reason, which is nice. No economy class seat is going to be super comfortable for long so the speed of the Q400 helps out with that.
I think the operator sees even more benefit from the higher speed than the punters. Shaving 10-20 min off every sector compared to the ATR adds up to a lot of extra utilisation per airframe day. The APU on the Dash seems to help with quicker turn around times too.