I have not had a problem carbonating the 5 batches of "big beer" I have made so far. When I say Big, I mean OG > 1.090. I have never gone more than 8 weeks in primary+secondary. I have used US05, Nottingham, Wyeast 1762 and 3787 which are all tolerant of higher alcohol though. If your strain poops out at 9%, or if you plan on bulk aging, you may consider pitching more yeast.