Same engine as per the current F10, but with revised internals including turbos & mated to an 8spd ZF gearbox. Output quoted as around 600bhp. The current engine already has a 600bhp variant as this was offered on the 30 Jahre edition.
Judging by the Timo video doing the rounds, they've made a significant improvement to the engine/exhaust sound as it appears nice & loud even in the softest settings. Certainly didn't appear to have any synthesised noise either.
Worth noting that as per the RS6, most F10's were outputting a fair bit more than the 560bhp quoted by BMW with many being close to 600bhp stock, so chances are in the real world these new 600bhp motors will be closer to 630/640.