In the absence of myostatin, roughly double the mass is resultant.
As a result of the muscle mass you would have a higher metabolism, but the lack, or presence, of myostatin is not a stimulant, per say.
If a person lacks one, or both, myostatin inhib genes, he will have increased skeletal muscle.
John Hopkins U has stated that they haven't yet observed any negative effective from myostatin inhibition / lack of myostatin.
Observers have reported almost no fat in those who lack myostatin.
Off topic / personal attacks:
Logan (flex89) is a type 1 diabetic that I help out. We are good friends, and have similar interests. Because Logan has a chronic illness, many folks his age, and indeed some here, do not have the depth of character, not the patience, to understand the challenges he faces minute to minute, hour to hour, and day to day. Things that are taken for granted by many are a major pain in the neck for Logan. Things like going to a movie, getting through a leg workout, or going on a bus ride.
It's most unfortunate that some folks, even though I've stated about things that really aren't the business of someone half a world away that doesn't contribute value on a particular topic, continues to make their attacks. At the end of the day, though, we do understand the source, and realize it is whatever it is, and take it with a grain of salt.
The ongoing personal attacks speak to the lowest element of these discussions, and while I know it's best not to acknowledge those sorts of people, for the sake of the other viewers, I'd like folks to know that Logan is a great guy, who face lots of challenges, and who is a good friend of mine. If someone half a world away wants to be icky, well, The Internet shields them and they're just icky.
While I'm frank, I spent 11 years in commercial broadcasting, and know pretty much what I'm going to say, what my basis it, and why my view is what it is. I seldom attack folks at a personal level, although if they're stupid, lazy, high-drama, I will almost certainly make them see that, as should be the case.
Thanks for making us feel special.
We'll continue to do our best to lead by example, and will not lower ourselves to some of the stuff here.