The difference between base and Z51 brakes are the rotors and the brackets. Front rotors are approx .6" larger while the rears are 1" larger and switching over to the Z51 set up requires the use a different bracket that you can get direct from chevy.
Also note that stock 12.8" front rotors are the same size for the C5 (inc C5 Z06) and base C6, but the C6 factory rotor was redesigned and will hold up better, at the cost of being 2 pounds heavier.
The calipers are also interchangeable between the C5 (inc C5 Z06) and the base C6, but the C6 are said to flex less and provide better, more even pad wear, resulting in better braking.
A couple of great cost effective ways to get great brakes on a C6:
Replace the aluminum factory front caliper pistons with Stainless Steel ones. This will prevent of lot of the heat in your brakes from getting to the fluid. It also has other benefits that are not as apparent. (Price about $150) I got mine from DRM, I don't know if there are any others out there. The caliper rebuild kit from GM is about $15 (new seals and stuff). It's not always necessary in most cases but it is cheap enough not to skip getting it. This is also a good time to change to a higher quality brake fluid with a higher dry boiling point. Brake fluid should be changed every few years regardless.
Replace the rubber lines with Braided lines. This will give you a much better feel and better control of the brakes along with better reliability. (Price about $120)
Go to a more aggressive brake pad. (I like the EBC Red front and back in the street for their quietness and very low dusting, they are also okay for light track days in a pinch. For track days I use the hawk DTC-70 for the front and DTC-60 for the rears, a lot of people also like the cobalt friction XR1 or XR2 in the front and either the XR2 or XR3 for the rears. Iíve yet to try the cobalt but will try them on this year. (Price about $200 for street pads and about double that for track pads.)
Install the C6Z06 front cooling ducts. (A slight modification to the inner plastic fender well to accommodate the different shape is needed). Price less than $20 for both sides and itís an incredibly easy way to get more heat away from your brakes over the stock set up. By getting the heat away it kind of replicates having a bigger rotor in thermal mass. They add life to the pads and rotor and prevent brake fade. Probably the best bang for the buck at preventing fade for $20. After 4 melted tie rod ends and a ball joint or 2 I wish I had put them on earlier. The tie rod ends are ridiculously close to the rotors. Since installing I haven't melted a tie rod end. (The ducts can even be extended right to the rotor with a little creativity using silicon hoses with not much work. E-mail me for details)
The torque curve change on the Z51 front rotor over the base is negligible and not a very good standalone reason to change to the larger size from a performance perspective. Different compound pads can do that for you. The larger rotor does gives you the ability to dissipate more heat, but that can be done with the cooling ducts. Besides I think the Z51 rotor will weigh a little more unless you go to a 2 piece rotor.
I'm not a huge fan of most drilled rotors, I like slotted or just plain flat for reliability. Most pads today have much less outgassing and don't benefit from drilled rotors on the track as much as they used to. Most of the brake pad outgassing is done the first time they are really heat cycled after the break in period. You need a lot of heat to have an outgassing problem on a good brake pad; Iím talking hard track time. (Always follow the manufactures break-in procedure; it's there for a reason)
If and when you do upgrade rotors, the 2 piece gives you less weight and also transfers less heat to the hub. I would do the above stuff before moving to a stock Z51 rotor. It will give you better more reliable and controllable braking over just the Z51 rotor upgrade.
If you do go to a 2 piece rotor one day I would definitely use that opportunity to go to the larger size. Iím still using 12.8Ē 2 piece up front with no brake problems yet. (I had ordered the 13.4Ē but the wrong ones came in and I didnít have 3 weeks to wait on the back-order for the 13.4Ē)
This is just my opinion. There are a lot of choices out there and most are confusing ones. These upgrades Ideas to ponder have come from trial and error on what worked for me in my situation with my budget. Since the above upgrades Iíve yet to have brake fade even in 100 plus degree July track days running pretty hard in group 5 at various tracks and I didn't spend thousands.
I'm not racing so lap time for me isn't as important as having safe reliable fun at the track. I'm not looking or capable of being the fastest guy around the track, but I might be the guy having the most fun.
Hope this helps a little.
Next on my list are the cooling issues with the radiator, the transmission and diff. Itís time for some cooling system upgrades. Iíll have to splurge for the Dewittís radiator I guess. That will probably solve most of my issues. Iíve already installed the Z06 oil cooler and it works great.
Whatever you do, Have Fun with that car !!