The 4.6iS is rated at 340 hp by BMW.

I heard the figure on the 4.8iS was 360 hp but it's also been quoted as having "13% more hp than the 4.6iS," which is 384. (340*1.13 = 384.2)

I know that the 4.6 is comparatively low-tech to the 4.4 in the 745i and 545i. The 4.4 has Double-VANOS and Valvetronic while the 4.6 only has VANOS. I'm not sure if the 4.8 is going to be upgraded to Double-VANOS and Valvetronic or if it's just the 4.6 with more displacement. If it's the former I'm more likely to believe the 360 hp number, if it's the latter I'd believe the 380 hp.