If you look at the limitations of any Robinson they've capped performance at 14,000. The 66 is a couple hundred pounds lighter with a turbine engine so naturally it will outperform a 44.
The performance charts in the POH are heavily underrated. If it doesn't hover OGE at 8,000 you should check your aircraft or pilot. We've flown them up to 14,000 with no problems. Keep ETL until you're IGE and you're well within limits. Tail rotor authority is also much better than expected.