Last year I put carbotech AX6 pads on the front brakes of my JCW halfway through the autocross season. I was amazed with the increased braking ability. Would I see much benefit by going with the same pad compound on the rear?

 

Probably not. The rear brake hardly does anything on these little cars. Especially with the little bit of braking that happens at an auto-x. It might change the brake bias too with different initial bite characteristics between the pads. Basically, if you're happy with how things are now then you don't need to change it.

 

Going with the ax6 on the rear will cause the rear brakes to bite harder and faster which can be good for rotating the car in tight turns. The 1521 "bobcat" compound will give you better rear bite than stock but not as much as the ax6. I would run the 1521 on the rear if it was mine because the compound will give a good mix of faster bite than the stock pad but still have good light feel for trail braking into sweeper turns and they are slightly cheaper than the ax6 compound.

 

There are at least 3 factors to consider:
1). Where you over-heating your stock brake pads before you switched to AX6's? I certainly did! Thus I went with Hawk HP+ pads all around (similar operating temp range as AX6's). Even though your rear pads don't do as much braking, they are physically smaller and you can and _will_ overheat them as well. That would be an argument for upgrading rear pads.
2). If you like how linear and progressive AX6 pads (or HP+ for that matter) brake torque application is on your front axle vs. OEM pads, you will probably benefit from the similar brake torque curve and feel on both axles. It will also help that both axles will be warming up at the same rate, and the feel will stay the same throughout the day. That would be an argument for upgrading rear pads.
2a). If you want to be really fancy, you can upgrade to some other rear pads other than stock. Ideally, that would be a pad that doesn't fade as quick as OEM.
3). Cost - Carbotech AX6 pads are among the more expensive on the market. That would be a (cheap-***) argument for compromising and going with cheaper or OEM rear pads.

HTH,
a