I have to agree with Johnc, Clint, others. Max out the capability of the stock system instead of the 4x4 calipers. The 4x4 caliper is HEAVY. More unsprung weight is not what you want. And it has a minimal amount of piston area increase. The pad area is greater, but the stopping power of the 4x4 caliper is not really that much (any?) better than a stock caliper, if they are both in good shape and USE GOOD PADS.
That last part is key. Get some good pads (I really like the Carbotech ones too - the performance street pads work great even when cold.) Add a good set of shoes to the rear, make sure the system is in tip-top shape, good fluid, and you should have plenty of brakes.
If you're going to upgrade, the fronts are where to start. And the only upgrade that makes sense is to USE A VENTED ROTOR. The issue with the Z brakes is fade, not clamping force. Add good vented rotors and the fade will disappear (or nearly so).
Dan Baldwin will tell you the only reason to go to rear discs is to get away from having to adjust the drums for the best performance often. SCCA will probably say that, and for better looks . I agree on both counts, but if money is tight, leave the rears alone, except for get some drums that are in good shape (not to large a diameter from wear) and the right shoes.
That's my opinion, based on years of watching this topic discussed here and elsewhere. I got stuck into the "got to upgrade everything" stuff on my car, and the Arizona Z Car Wilwood kit was offered to me with little use on it for half price, so I bit.