I'm curious to know why you're so hot to know this. Until Hardware 4 comes out, your car won't have the processor to handle 5.0 megapixel rates. So even if you have 5.0 megapixel cameras, they're running at 1.2 megapixel rates. If you want to try to shoot holes in this idea, transfer some dashcam footage to a computer and see how many megapixels are being recorded.
The only reasons I can think of to install 5.0 megapixel cameras now are:
1) If they're as cheap as the 1.2 megapixel cameras, use them. This is a bit like switching to matrix headlamps without taking advantage of their abilities.
2) If Tesla plans to upgrade all cars to Hardware 4 eventually, put the Hardware 4 cameras in now. It's cheaper across the board.