It's quite a long way 'till the next generation will appear and these are just rumors.
If they will release an M Performance petrol version (as 750d is available with the LCI) this will be based on 750i and will compete with S8, following the same formula: less weight and around 500hp.
I don t think the numbers are fixed yet as you can't present a new product, expecting to be the best, but with important less power than the rivals.
For sure it will be under 760i and I guess it will be called M 750 Performance.