Has anyone replaced the rear brake rotor and caliper seems like the Oem one is pretty small and pointless. Any help is much appreciated.

My opinion , try first aftermarket slotted or drilled rotor ( you'll get heated arguments on which is better) most mfr's have combined slotted AND drilled actually to appease BOTH arguments !! Then go with a 'grabby' , Hi-perf pad , owners w/same car reports are helpfull - not critical . Out of the dozens of combos out there , whatever - you should get a noticeably improved performance , even if your choices not 'God's choices' .

I would just stick with the oem style, most of your braking is done by the front brakes anyway.

Nah , sounds like OP wants KZILLA-stopping power , wants someth'n different , wants someth'n BETTER , and they can get it . From better aftermarket , all the way to maybe larger rotor and bigger & better/improved calipers.
Gotta start somewhere , I bet they'll eventially get'm ALL around to the fronts too.

KILL-ZILLA-STOP 'N ON A F 'n DIME

I already went with drilled and slotted rotors. But I was wondering if any of the other platforms that offer bigger rear rotors and calipers would fit the Gen 7 Maximas.