So I am doing my exhaust this month and am wondering a few things.
My plan is to get rid of the 2 rear cats, will this cause any problems at all?
I am deciding between running true dual 2.5" or a single 3".
After ditching the 2 rear cats, would there be much of a gain by trading the fronts out for high flows?

 

Removing/replacing the cats, the only benefit is sound. If you want the most sound, swap all four with two high flows. A little less just delete the rears. As long as you stay 2.5" pipe where the cats where and correctly place/modify the O2 sensors there won't be a problem, but it is strongly advised by tuners that any modification to the cats should be accounted for with a proper custom tune.
2.5" dual or 3" is really your call on how you want it to look. You can find mandrel bent tailpipes all over the internet to stay away from a shop's crimp or stretch bender.