Just noticed your Avatar is the picture I took of me and Oleg (can't blame you for cutting me off it!).
The smoke effects really don't make a huge hit on FPS as it is still a 2D sprite, just remapped and modelled to look more authentic. The flame could be toned down a bit but it really is a case that it adds so-much to the immersion especially with Machines now having to be powerful enough to run CloD!
Check out this video to see what difference the smoke and flame effects make to the 10-year-old game engine!
Cheers, MP