Most RK seats are interchangable with the SG. The SG has a long dash panel, so you wouldn't need the dash panel extension.
It should be ok, but sometimes you just don't know until you fit them up. Most 02-07 RK seats fit 06-07 street glides. However, I had a Mustang seat on my 07 RK that apparantly did not fit a 07 SG. Never did find out exactly what the fitment issue was, though.
Also Corbin is kind of fussy about what years their seats fit. I think they seperate out the 06-07 RKs from the 02-05, while most other seat manufacturers don't.
Bottom line, you might not know for sure until you put it on.
Which is the seat in question?