Hey, Not being a frequent visitor to the site I always feel i always put my 2 pennies worth in a bit late but here goes...
I've painted the callipers on all my cars in the past and call me agricultural but I've always used good old plain and simple Hammerite, and it seriously lasts.
I've know other people who've bought the branded calliper paint and it doesn't last any time at all, so they can be a little hit and miss.
BEST thing about hammerite is you don't have to worry so much about surface preparation. I give the callipers a quick blow down with the airline, quick wire brush on main surfaces and slap the paint on. I do use brush on as opposed to spray on, as spray paints by nature are thinner and so will give a less durable finish unless of course you build up lots of layers.. but I've not got the time for that.
Although, I've always shied away from red and always gone for Hammerite smooth silver finish with calliper details emphasized in another colour. You can see on the pics below in the z4's case I did the springs in light blue. They look as good as the day I painted them and that was 10 months and 12k miles ago.
My old car I repainted after 3years and a good 40k miles, they probably would have cleaned up well but was easier to repaint
I think silver looks more OEM or black on black.
black would suit your lovely Z too!
