Well sure. When I asked the question, I knew that I'd get that answer . But I got some others too. Mostly based on conjecture, not actual experience with having tried the 20-minute test with and without the 5 minutes. I guess I should have expected that as well. So yeah, I'll do the proper test some time*. By then, my previous results will be ancient history, so I still won't know how much the extra 5-minute effort mattered.
Mostly I was just curious - thanks for humoring my curiosity for a spell.

*The fact I haven't done that is at least as much a time issue as an aversion to pain: quite consistently, my riding consists of one-hour-ish rides during the week (lunch rides), and a single long climby ride on the weekend. I don't want to put the test into my long ride, so I have an hour for the test ride. The way roads work out around here, the actual test is pretty much all "out", which means I still have the "back" stretch of the ride to do after the test. Doing just the 20-minute test, that fits OK into a one-hour ride. An extra 5 minute effort + recovery starts to stretch it.