Yes, it varies a lot and it also has a lot to do with ambient temps, how long you've been driving, and how much load the engine is under. I have a stainless exhaust here that was on a car for quite a while and it has turned a light straw color most of the way back which means it probably reached a temp of about 500deg F more or less at one point in time or another.
Bigger pipes will definitely run cooler than smaller ones but the exact temp would be hard to pinpoint. If you are concerned about the gas tank, as long as the OE shield is in tact I would not be worried. If the OE shield is gone then I'd heat wrap the portion that goes past the tank.