After bringing my awesome negotiating skills to bear on my LBS proprietor ("how much?"... "shout me lunch today and its yours".... "OK then, pizza?"... "yeh OK") and it was mine. So when I got home it only took 20 minutes to swap over the front end from 20" to 26".
The first thing I noticed is that the front end went up (really?... naaaaah!

I went up to the shops for her 26/26 maiden voyage, mainly because I get to go up the T-way, which is nice and smooth and has no traffic but for one bus every 10 minutes. It was immediately obvious that there was tonnes more feedback from the front wheel! I could tell exactly what the steering was doing, by comparison the 20" was more of an educated guess. The bike path from my place to the T-way is a bit rough but the bigger front wheel made it significantly smoother...... Then I hit the smooth T-way and opened up the throttle, and that's when the fun really began!
She immediately accelerated up to 36km/hr and I could cruise at that speed with the same effort that used to get me around 28 - 30. The handling at speed is definitely less twitchy, and is easier to hold a line around fast sweeping curves. That was on a nice flat and slightly undulating stretch.... then I went up the shallow-ish climb leading up to the shops which was just as slow as ever

And finally, the deal clincher! last night's ride home was head on into a very stiff west-sou-wester as a cold front swept through, and I was comfortably cruising past everyone on uprights like they were standing still (ok, some of them actually were standing still) but seriously, what a huge difference! with the 20" I could feel the wind on my legs, chest, and face, but with the 26" I feel the wind predominantly on my legs but the rest of my body is in my leg's wind shadow and I think that made a huge difference.
So, what started as an experiment to satisfy my curiosity has completely transformed this bike. I'm amazed what a huge difference it made, and it's really got me thinking about other improvements that could make a significant difference..... tail fairing maybe?