No, 6 is not too much -- not to us here!
When I go to NY, I plan my trip around the number if shows I can afford/squeeze in! So. In 8 to 10 days, I'd be seeing 10 to 12 shows.
Your list looks great. Shows to consider adding:
Something Rotten
The Visit
Hand to God
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.