I would prefer future M models to maintain current horse power and get lighter rather then get more powerful and more heavy.
Reason being lighter cars corner and brake better.
Also if you look at the mclaren f1 I think that has like 600bhp and only weighs around 1000kg and from the below video it appears to be faster than the veyron (1000bhp) until they go over 240kph (150mph).
Ok then veyron trumps it, but still closer then I expected.