It's less of an improvement than going from 2MB->8MB, that's for sure. The 16MB drives are generally a little faster. Most of the 16MB drives are a full generation or two newer than the 8MB models, so it's hard to say exactly how much benefit is from the bigger cache and how much is just from general improvements to the drives' mechanics or firmware (and higher platter densities).
I wouldn't bother upgrading just for this, but if you're buying a new drive anyway you might as well get the latest and greatest.