Not too extreme at all, no. I played in a little campaign with my gobbos a little while back and the organiser had made a few houserules, like letting all-goblin armies take big bosses as a 2-for-1 choice. So my army's character selection was:
Goblin Warboss
Night Goblin BSB on big Squig
Goblin Big Boss Wolfbomber
2 Level 2 Shamans
Not abusable at all, but let me use a fair few more toys to help keep up with the competition.
Mind you, I still doubt that any % restrictions will overrule the blather about army composition and slots in every army book out there.