i dont know about for a SBR.. they dont have alot of pull with me..
But M/G's and silencers are DEFINATELY worth it.. they kick ass!
and if its something you will enjoy, and you have no criminal history, its pretty much a slam dunk.. takes a long time.. but its not real tough..
Ive actually been considering registering my T/C Encore as a SBS.. but thats a different story altogether, as i would like to use it as a pistol 20 gauge..