Well, basicly, that's the way I do it as well, only I DON'T use hardware particles, unless I need REALLY small streaks...
To create controllable heat distortion
effects, I mainly do this:
Say the shot calls for hot exaust coming out of your jeeps' tailpipe, and the vehicle and camera are not moviing very fast, if at all.
I would try this first:
1. Make a small emitter inside the tip of the tailpipe.
2. Make it emit software cloud particles mapped with a basic lambert grey shader with little, or NO transparency.
3. tweak the particles with a turbulence field etc. to get it to act correct.
4. If you want to, tweak the software cloud particles size with a radiusPP() creation expression like this: (you will have to add radiusPP as a new attribute)
particleShape1.radiusPP = rand(1,3);
5. Render the particles out using the software renderer.
6. Using After Effects, use the particle render sequence as a displacement map sequence for your background
7. Use that sequence to also add a little blur to the background as well for spice
Hope that helped...
If not, lemme know what more you need to know, and I'll try to help out
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS