I guess I don't understand the distinction of grading things on an onsight vs redpoint. Yes, when I'm attempting to onsight a climb it often feels harder but I'm cognizant of this and can still categorize the climb relative to other climbs in the area. For me, Rolling Thunder is easier than House of Pain, Vicious and Chainmail but harder than Screaming For Change, There & Back Again, and Orion. Therefore (in my mind) it's about a 5.10d.
Chad