you dont need to upgrade the rear calipers unless youre really serious about the track! just put the best pads you can get in them and theyll be fine (I put QFM A1RM's in mine)
but about the painting, I was looking into it as well, and this is what I found:
the professional way that brakes are painted is a powdercoating, this is how all of the big branded calipers come from factory including our redline brembos - it is incredibly heat resistant and you get that nice smooth finish, however you need to get a brake specialist to take apart the calipers and re-build them after with new seals that you will have to track down yourself (maybe from holden but who knows), seems like a lot of mucking around...
on the other hand you can just get them painted, but its not as resistant to heat, takes a lot more prep work, and cant guarantee that professional looking finish, seems a bit risky!
you will also need to track down more brembo stickers in heat resistant material in order to put back on after painting, another muck around - I was only able to find these online and from dodgy looking sites!
either way will require the calipers off the car, hence the car out of action for several days!
we may have to live with the silver!