I don't know about a brake 'upgrade' for that model. But often, just getting new fluid and having the system bled properly can make a big difference. Especially if you combine it with some braided brake lines and sporty pads.
What's the issue with you current brakes: initial bite, braking power or over-heating under hard use?
i go to alot of track days and i know from experence that bmw standard brakes tend to heat up and sufer from bad brake fade. the only perofmance upgrrades i can find are brembos at around £2000, but i can get 6 pot calipers with discs and pads from £700 for E36's and E39's.
when the weather gets better im going to try the brakes off my e36 on the e38. if that doesent fit, i'll try and and get a loan of a e39 caliper and braket or get a scraped one and try that.