I do not have the AA pack for mine, but one problem with alkalines is that they choke on high-current loads. So your TX power will be limited. This is not a problem for receiving, though. That is why custom lithiums are used.

And as for the aftermarket battery packs, check the specs. The 1800, if I recall correctly, is thicker than the standard battery. So it will work, but you will have to remove the belt clip. I have not looked at the pack in a year or so, so maybe the newer ones are the same size. Just something else to be aware of.