Lets just say i bought a cheap pair of OEM regular G35 Coupe front calipers and I want to fit them to the rear Rotors ....

a.Would it cost much to retrofit them to the rear?
b. would it make any difference in stopping power ?

Cost would probably be $2-300 if you give the machine shop the exact specs of the brackets that will need to be made if you can do this yourself. Expect to spend 40 hrs+ getting the brackets right. If you can't do it yourself, then expect to spend a substantial amount of money doing this.

What will be the result? Significantly WORSE braking performance than stock, probably to the point of the car being dangerous to drive due to the change in brake bias

^hmm, thought it would be better, since the calipers in the front are bigger ,

Just a thought tho, not ever going to do this !!!!!!!!!!!!!!!!!!!!!

There's a reason the calipers in the front are bigger, and that's because they do most of the work, and the brake bias is set for that. Putting bigger calipers on the rear will increase the force applied at the rear and upset the carefully cailbrated overall braking balance (bias). This will result in longer stopping distances, and potentially set up early rear tire lockup, which would cause frequent spins.

The companies that sell BBK for F/R spend a lot of time and money getting it right. This would work on a Track Car. One that all the ABS and VDC had been taken out. Then all you need do is put a bias adjuster for F/R adjustment. On a street car with ABS/VDC, it really is not advised to experiment with your life and that of others on the road.

This ^^^^ is impressive, knowledge and common since applied to a subject
and walla you have the correct answer. If the object of putting larger
calibers on is to impress buy caliper covers, they glue on and someone might
actually think you have a BBK....
Gary