Rear breaks on Classic
 

Is there any advantage upgrading rears from solid to vented disc? Front is already 4 pots.

Hi
A vented disc will run cooler , so if you are already overheating the solid setup then yes it would be better going to a vented setup , but to be honest , if your going to go to the extent of swapping calipers and disc , you would see more benefit of fitting the 2001 onward WRX 2 pot rear brakes.
Cheers Ian

Thanks Ian I do know what you mean.
The thing is that I have got Legacy with vented discs that got covered around 500 miles prior to accident so no cost involved in that case just man hours. I do not know if Legacy calipers have the same size pots this can be verified later. As far as I read there is not big improvement in braking reported when 2 pots installed on Legacy.