Upgrade to iOS 9 Final from Public Beta?
As no answers are forthcoming, I'll report here my findings. 

On 2015-09-16 , before iOS 9 GA, I removed the "Beta" profile from the iPad and rebooted. After reboot, software update says:


  iOS 9.0.
  Software is up to date


The version of the software is 9.0 (13A4325c)

So it looks good. There's an oddity, though. When I've rebooted (and hard-restart too to be on the safe side), every time I unlock the iPad I still get a notification saying:


  There's an update available. Update the iOS 9 beta version. 


I hope that when 9.0 final comes up, the update check will pick it and I'll be back into mainline versions. I'll update here when things change. 

Edit 2015-09-16 19:39 CEST: Apple has just released v9.0 final and going to software update in Settings I get prompted to upgrade, so it looks like I'll be able to get back to mainstream. The notification after login stills says the same. Will report later when I have the chance to upgrade.

Edit 2015-09-18: After installing the 9.0 update, and the subsequent reboot, the device says its updated to 9.0 and I no longer get the prompt to update -- so all seems to be fine!

Edit 2015-09-25: Just a final update to confirm that after a few days running, all seems to be fine. I've just got the prompt to update to 9.0.1, so I seem to be fully in "mainstream" now.