#1. Corona colour correct is not linear, if you want to get the same result as with invert in Max's bitmap or output nodes, then you can use curve adjustments in CCC.
#2. If you want metal, then switch Corona physical to metal node. 2-3 IOR is not metal.
#3. what denoising you are using and what image filtering. If that's Corona HQ denoiser, then filtering also should be set to HQ, otherwise it will severely underperform. I didn't notice dependency on material type, but maybe that's because i almost exclusively use Corona physical.