I posted over on the MS Forums as well and basically got a "yep, this is how it works for me too", as well as a work around. The work around (to not explicitly have the parameter names in the exec query) works, but doesn't actually prevent us from having to make a bunch of changes, and may not even work in certain situations.

So I've opened up a bug here... https://connect.microsoft.com/SQLServer/feedback/details/795002/sp-describe-undeclared-parameters-case-sensitive-despite-collation

Thanks for checking it out and confirming we aren't crazy!