I've got full brembo calipers with Z04 and HPK discs now and it's other wordly but the biggest difference I found whilst building the bits up was changing the master cylinder to a Brembo RCS (surprised someone above didn't think it was much difference but I guess its down to other factors as well perhaps discs and pads?). If I was to upgrade on a budget I would recommend good pads and a master cylinder, then discs. There's not much point buying average pads or discs though since they are just a cheaper way to replace the stock ones but not 'better'.