Not by much at all. A 12AT7 will put out a gain nearly comparable to a 12ax7 in practice. The much larger internal plate resistance of the AX7 (rp) will ensure that it will never nearly reach it's potential MU of 100 in most applications. about the most you will ever get out of a 12ax7 in most configurations is a gain of about 60-70, and it's not even pushing that in the PI spot, where it also has to drive a somewhat heavier load presented by the often 100K-220K gridloads of the power tubes (often mis categorized as bias feeders). high Z tubes like the AX7, are not great at this.
on the other hand , an AT7 has a much lower rp, and will be able to drive output tubes a little harder as well as heavier loads, and a offer bit more headroom as there slightly lower input sensitivity will allow them to swing more. You should however set the amp to more effectively use them though if you want to gain the performance.
