To reduce the level of misguided answers make s
This is semi-related, but note that Visual C++ does have the ability to do cross-module optimization, including inline across modules. See http://msdn.microsoft.com/en-us/library/0zza0de8%28VS.80%29.aspx for info.
To add an answer to your original question, I don't think there would be a downside at run time, assuming the optimizer was smart enough (hence why it was added as an optimization option in Visual Studio). Just use a compiler smart enough to do it automatically, without creating all the problems you mention. :)