My old Alfa would backfire if I really let the engine over run. Heck, my 2x160cc Go Kart does it. I added backfire sounds to a little driving game I was working on and it brought the game too life - sadly the physics killed it again but it was only a few days playing around to make it, the difference with and without is quite something.
Pressumeably insim provides feedback on revs, if so this could be used to monitor engine drop offs which could give reasonably realistic information with which to do the backfires in a mod that just detects sudden rev cut offs and adds the odd backfire sound - maintaining sound quality might be an issue because the OS will duplex the audio between two applications, it would have to be tried to see if it's good enough.
If you want flames in the game too (which dont add as much, it's 'feeling' your own car that makes the biggest impact) then a much more complicated mod is required.