I've got 280ZX calipers with R4 pads and turned down '84 Z31 rotors on the front, stock drums and OEM shoes still in the back, driving on Hankook Ventus R-S3 tires. Had a blast last year on the track (30 minute sessions) without any braking issues. I am not driving to the limit though, and my setup is very front biased.
This summer I am going to install rear 280ZX discs with '86 Maxima calipers and R4 pads, as well as the 15/16 master and 280ZX proportioning valve to see if I can equalize things.
Although I am happy with the brake upgrade options, I've run with faster people using well-maintained bone stock brakes. It is amazing what good fluid and pad/shoe materials can do on the track. I think there is more bling and less real performance with "upgrades". I don't mind admitting I really hate drum brakes though.