Custom fitting brake caliper and rotors from other cars
 

I'm looking to install factory brembo 6 piston front and 4 piston rear calipers from Cayenne to rx8 or 4 piston ST-40 in all 4 corners. mostly to give it little better grip when trying to stop from above 150mph. I have stoptech in the front, but they charge ridiculous amount for matching rear set. I will have brake distributor for balance.

How hard is it to make brackets to fit the calipers?

I am going to buy the brake rotors new to fit my bolt pattern. I am thinking of putting caliper on the rotor, give it pressure with air compressor, and make adapter out of solid aluminum unless steel is needed... has anyone done it?

Making caliper brackets isn't necessarily that difficult. It is however time consuming and the measurements need to be fairly precise.

You could potentially do it that way, although you'd need to ensure you're measure two or more times and cut once. The reality is that you could do it a lot of ways. A good quality aluminum can be used for the adapter but most would go with steel. If you think you can do it, give it a try. Just be aware it will probably take a lot more time than you think to do it and to go easy when you're checking your work.

All of this being said, perhaps a more aggressive brake pad in the rear will give you what you want?