If I hadn't got the TAROX's and was thinking about an upgrade then the RS4 upgrade would be near the top of the list at £1,350.
Brembo do a Junior upgrade at around £1k plus fitting, but the discs are still a little on the small side.
R32 upgrade is a value option, but is a heavy item and still only single pot.
AP's are a very well respected brake system, but they too are a little on the expensive side at around £1700 plus fitting for RR's set-up.