Flown on the Chipfryer RJ's many times.....
I am over 6' tall and do not have any problem with the seats or headroom. The baggage bins are smaller under the wing but the headroom under them is constant throughout the aircraft (it would look pretty stupid with a step). There is reduced headroom in the cabin under the wing but this is not noticable and I have never banged my head on the roof lining. The seats are electrically adjustable so the cabin can be reconfigured to give a four abreast business class. I'm not sure but I think the last two rows at the back are four abreast not six. I don't often sit in the back, what's economy class?
As for most operators using 5 abreast, I can think of only one off hand, and thats Mesaba, and that is only because of the pilots scope clauses in the US (they operate RJ85's with less than 70 seats).