I absoltely love the idea of this, hell I'd even join an SG if it was RP only..
BUT.. and this is a big one for me...
Playing MMOGs is a hobby for me, so I tend to spend a significant amount of time, I'm a hardcore player for the time I spend playing, though I don't do it powerleveling. But the amount of time I spend would unavoidable place me outside of your level cap.
Got this game two weeks ago and I have a level 15 scrapper, level 10 healer/controller and the scrapper is the second build (first got to 9), now I know the first few levels are the quickest but nonetheless....
Why should the amount of time I spend playing restrict me from being in the group?
I mean, even someone that is a powerplayer and spends alot of time doing group hunting could still fully be able to roleplay that, and do so significantly well.
This idea will have a severe difficulty getting off the ground or more then a handful (therefore be almost useless to any that wish to RP with others), as most will be discouraged by the idea that they may hook up with an awesome group on Monday, roleplaying the whole while, and hit the level cap by Tuesday...
So what? They can't level til next Monday? So... they can't play? Or HAVE to play another character?
How is that roleplaying in itself
IC - "Hmm it seems that I have been learning how to control and manipulate my latent abilities well beyond what I should be capable of... guess I need to go on a vacation...."
Kill the level cap please

I'm not a power leveler, but I tend to play from 3-4 hours a night, doing missions, having fun, but severely miss the RP environment
The amount of time you spend playing should have NOTHING to do with being in a RP group..
That'd be like saying.. hey you can play D&D (the old paper and pen version) but if you do it for more then 2 hours a week... your not roleplaying anymore because you spend too much time doing it...