Teams have the option to still run a last year spec V10 at reduced revs. The pick up better from low revs, but I'm sure as the teams progress though out the season the V8's will rev harder to produce more speed than a V10 with reduced revs. Don't forget, they can run a V10 this year, but next year they'll be a year behind in a V8 engine development.
The end result is a croc of sh*t from Mister Mosley. Cost savings in F1, my butt! The switch to a V8 means teams had to spend a lot more money developing the new engines. It was never as simple as the FIA said (chopping 2 cylinders off and tada!) because V8's aren't as balanced as V10's.
I can understand why Michelin's leaving now. Too much politics, not enough racing.