You could use UA strings to serve the shims and workarounds, I suppose, but you could also use client-side feature detection to do the same thing more reliably. If certain features are missing, you load in the extra code. If not, you don't, and page load time is unaffected, except for the tiny amount of code required to do the feature detection. In either case, what you're doing is more graceful degradation than progressive enhancement.

But the idea of progressive enhancement is not just about supporting everybody's browser so that you will get more conversions in the short term. It's about the long term. Targeting the majority and saying "screw the fringe demographics" is what led to Microsoft having control over web technology years ago, and it's important to our livelihood to prevent that from happening again.