On his D, he's been an adequate defender the challenge has been maintaining intensity while carrying the O - could he make a Ray Allen type change? Reduced minutes and smaller O load - would he be able to channel the energy into his D? He clearly has the physical tools (assuming knee is back) and know how to defend...
Yes - I think assuming he's healthy, he'd be better than Posey. He's as good of a shooter, better at everything else. The only place Posey was better was D intensity, which I think T-Mac could make up for with his overall versatility on O and reduced minutes / load to increase defensive output.
Again - I'm only a fan after you have fixed the big rotation and after trying better options, if you end up with Nate and TA as your backup wings/guards.