I'm considering making a switch to disable Bloom. Trying to work out the calculation count is a little hard, I think it's around 130 calculations per pixel.
What will most likely happen is the GPU will utilize all it's shader units, which is basically the whole programmable pipeline.
You might be better off to try the Lottes shader for Mame if you just want to remove Bloom. I bet it get's just as hot though.
The original Halation did a heck of a lot more calculations, I'd guess from memory, approx 90 or more extra per pixel.
EDIT::
additive_bloom = 0
bloomAmount = 0
Bloom is always on atm. So setting bloomAmount to 0.0 just means it will be doing math with a 0.0 amount, mostly addition and multiplication.
additive_bloom = 0.0; will switch the final bloom calculation to a slightly different one where it isn't just adding the worked out Bloom to the existing screen pixels. Not worth turning off to be honest, unless you like the look of the different pipeline.
I'm still learning the GLSL, it's a pretty big step up from what I've been studying at University, but at least I can think like a programmer and work things out

I'll see if I can sort out the AM version of Lottes tomorrow. It should be fine for the Mame version though.