I bought two ButtKicker Mini LFEs for my driving sim rig. I purchased one for the front and one for the rear of the rig. Thought about getting four, but with headphones on and the sound being directional (left vs. right), the brain totally gets fooled into thinking the vibrations are coming from one side or the other. The difference in immersion with the LFEs is simply astounding.
I initially had too many signals triggering the LFEs and overheated them (they bounce back after cooling down). I subsequently got very selective on which signals to activate along with their levels and everything's running great now. They do get warm, but by no means hot. Being highly selective on the telemetry signals also brought a lot more realism to the driving experience.
No regrets in not getting the bigger LFEs. I've got the amp volume turned down considerably and it's more than enough to get the results I was looking for.