Is GSM telephony feasible on the Samsung Galaxy Tab 8.9?
You write that the 8.9" tablet has support for GSM telephony but that it is not activated out of the box - and that suprises and concerns you.

Well, for many of us, the original 7" Galaxy Tab is the same: the hardware supports GSM telephony but the North American firmwares have no support for it.  Presumably this was done at the behest of the N.A. carriers who want users to use the tablet as a second cellular device - not as a replacement for a phone.

As a result, I had to root the Galaxy Tab and load a European firmware on it in order to activate its GSM telephony.  Once I did this it worked perfectly well as both a phone and a tablet.  (Though it does seem rather slow by today's standards, and it won't be getting ICS.)

It sounds like that is the same situation that you would face with the 8.9" tab: the fact that the GSM telephony was not supported out of the box did not mean that it didn't work well - but you need to be prepared to get your hands dirty to 'activate' this functionality yourself.