ALL tanks that are in the FW kits are tempered. EVERY panel of the tank is tempered, sides, back, front, bottom. TRUST me, I found this out the hard way, got half way through the hole and it exploded.
Did you find out for certain that the sides are tempered apart from the experience you had? AGA's technical department gave me the info I posted, so I'm wondering if it's possible that you just had a bad drilling experience.
I've had non-tempered glass explode the way tempered does when drilled before.
At the same time though, the rep I was talking to was referring to individual tanks, not those included with kits, but I would expect that what they sell in kits are the same tanks as the ones they sell individually.