Eighteen Poison.
18th. actually by doing the head to breath more better, the more the hp's generate. remember race cars engines, most the work is on the head after the engine block.
so, back to the cams, the M52 uses the single valve springs, conical type. this because BMW wanted good fuel savings (lesser friction movement), so in order to stay at top end speed of 250km/h, at about 6600rpm, you will need good heavy springs in order the 'springs' to not float at high rpm.
so, use the M50 non-vanos springs set up, they use the double springs as the Non-Vanos M50 has hot cams (very similar to the S50 US M3). use the seat valves and caps, etc.
this will help on the high end.. by the way, it wont happen on the e38 !!.. must be crazy to be that rpm daily.