Any question answered...

That's between a standard 32 spoke wheel and an 808 at 25km/h, front wheel only.

2W in a 25 might get you from 1:00:00 to 59:52, although at that speed an 808 gets you more like 8W, so about 30s.

So, assuming I was to achieve my target 740k (ha! Well, I've got nothing else to aim for), using an 808 over a standard front would get me 4-5 extra kilometres. I'll say "it's not worth it" now and then miss some milestone by 1k and then regret using a standard wheel forever..