@evildictaitor:You can't prove something will improve productivity to the point that justify the additional cost. You can say that there's feature here and there that will improve the productivity, but try present that to upper management and see if they can understand!
But trying to quantify the improvement usually is wrong too. A productive team is used to current tools, so additional tools - except a long wanted one - will add little to productivity. A poor performing team won't get much improvement too becasue their problem is on another level that's not something development tool can improve. So the estimation have high chance to not look impressive enough to justify the cost, or just plain wrong.
So the XP support is really a issue for developer needing a new tool, especially since most developer is still using WinXP as development OS in companies, the cost of moving to VS11 will be "the cost of using VS11 + the cost of migrating to Vista+ systems" in their eyes. Whoever want to push buying this would certainly need impressive report to let the upper levels approve the budget.