@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!
I do it all the time. Upper management always wants you to be more productive for low cost. What they don't want is spending money for stuff developers don't want, or spending lots of money on stuff you don't need. If you say to the CEO of your company "This tool will allow us to make your products faster, safer and ready-for-market sooner" then he'll get out his check book.
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.
If that were true, why did you ever leave Visual Studio 2003? And what possible scenario could there be where your CEO would ever allow you to buy VS2011, XP-support or not?
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.
Your CEO doesn't care about XP or Vista or anything. she/he cares about profit margins. If 90% of your customers are using XP, you're probably not going to be able to upgrade to VS2011, but if 5% of them are, and your team will be 10% more effective with VS2011 than without, then the CEO will happily ditch those customers in order to reap the 5% marginal improvement.