jcflorida...this is not a simple answer because the vehicle has the head/curtain air bags which set the dimension outboard at the door headers. Fairly quickly you transition from headliner to sunroof opening and once you get to the opening, you may actually get slightly more headroom depending on where your head is with respect to the sunroof. Best bet would be to sit in the Acadia or Outlook...they will be similar (but not identical)...if you're within about an inch of touching the roof, there might be a concern...if it's more than that, you should be ok. Biggest difference from the other two is in the third row (about 1"less in Enclave)...second row is about .3" less in Enclave.
correct zman and linuxtwit...the roofline in the Enclave is slighly lower. It's approximately the same at the windshield, but slopes downward about an inch at the full rear. There is also a bit more "tumblehome" (inward contour of the body above the beltline) on the Enclave so there is slightly less shoulder room in the third row as well.
This has brought up a point that I had not even considered - that the Enclave might have less interior room. We were quite pleased with the comfort of the third row in the Acadia. Has anyone sat in the Enclave and are any of these dimensional differences noticeable from a comfort standpoint?
Interior room for Enclave is .2cuft less than the Acadia...and it's really all in the rear...and due to the tumblehome. We've had 6 footers in the back seat and unless they are very long bodied, head room appeared to be fine. It is, in fact, VERY SLIGHTLY less than Acadia so if you're planning on seating 6 footers regularly in the rear, the Outlook/Acadia will have just a bit more room. Hope that helps zman!