If you're just using local values, or storing them in an array, then, yeah, value types might help. That's a likely scenario for something like storing arrays of vectors.

The spot where you take a hit is when you've got a scattering of individual objects that are being passed into and out of functions. There, the extra data copying has a tendency to outweigh the other benefits. And that's a more likely scenario for most kind of the kinds of business apps that .NET and Java are typically used for.