Upgraded gateway on 300 mbps plan?

We've had xfinity for not quite two years; the 300 mbps plan using the XB6T gateway device.  In researching a specification question my son had, I find a lot of references to this unit being somewhat obsolete.  I also find multiple places where it says a user can upgrade to XB7 simply by taking the old unit to an xfinity store and swapping.

Question 1:  Is the XB6 capable of providing all the performance that I need?  Why is it referred to as being obsolete and/or "economy", etc.?

Question 2:  Can i upgrade by swapping at an xfinity store?

Additional question:  What level of speed would I need to move up to in order to automatically get a better gateway?

Thanks.