This might be my inexperience with guns showing, but if you think there is enough heat to cause issues with leather/rubber grips, I'd be concerned about there being enough heat to cause a round to go off on its own.
That being said, if you keep moving (as in, don't sit idle for 10-15 minutes) and the holster isn't pressed up against the exhaust pipe, I think you'd be ok. The pipe is going to be a lot hotter than the engine casing (think 500 degrees as opposed to 250 (I'm pulling these from my tushy, they aren't real or scientific)). So again, unless you keep it pressed against the engine for a long time or you press it against the pipe for a shorter time (minutes against the engine, seconds for the pipe) the holster should be fine. You can test the times with a crappy pair of shoes. if it melts, it has touched it too long
As for the bulk, I can't tell you. Never worn an ankle holster myself. I imagine if the weight is more than a few pounds it might catch you off guard if your bike slips on gravel and you have to catch it. The added size might catch on things or push your foot to the edge of the pegs. Both of those would depend on what gun you are carrying. And, like I said, I'm not really experienced, so my advice might not be all that grand.
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