I was 165lbs when I bought mine, if I put the fork down I could be there again
If they are too stiff you can have them cut down or just remove a rivot from the cuff to soften them. You would have to be willing to put up with cold feet sometimes as there isn't a lot of cushy insulation in there. I use boot gloves when it is below 15* and I haven't frost bitten any toes yet. Unbuckling on the lift helps too.
As far as ability, I'd have to defer to others here. Like all race stuff, your movements get transfered without much translation and feedback is immediate. I think they helped me ski better. I can't see ever going back to regular boots.