Around 190bhp/ton, the facelift has 260bhp so more around 200bhp/ton.
They're great drivers cars, for the road especially.
I couldn't help myself, I went out on a test drive with a z4 M roadster the other day with intent to buy but it disappointed me! For a road car, the 3.0 is perfect, the M is just too much for British roads, I thought this when I had my M3 but this test drive just confirmed it for me. The acceleration is savage, there's no other way of explaining it, the power delivery doesn't work on the road IMO.
I got in mine afterwards, drive home expecting it to feel like a piece of crap, to my surprise it was lovely. So smooth, plenty enough acceleration to see off most things on the road AND have fun without loosing my license. It doesn't have the thrill the M does at 10/10ths but that's to be expected, the extra running costs aren't worth it.