Many thanks to DA for the pioneer work on the 20-16T conversion.

I did it 2 weeks ago.
Motor assist up to 21mph (Up from 16mph).
Average speed now 17.5mph (Up from 14mph)
Speed up my (moderate) hills has improved, which is a relief.

Best of all, the motor gives assist at normal cycling speeds, rather than easing off just as I get going.
I now arrive fresher, and am actually using more juice from the battery. Previously, I was using about half the AH in a 10AH SLA pack when on an 18 mile round trip. Now I use half the AH from 2x10AH packs wired in parallel.

I view this as a good thing. The bike is doing more useful work for me.

Before I did the conversion, I fitted an ammeter as I wondered how the controller regulated power vs speed.
Reposted from New Members forum:

On a standard 24v 20tooth Izip (at full throttle):

MPH.......AMPS.....Watts(calculated)...16toothMPH(calculated)
<10.......25..........625.......................<12.5
11.........22..........550.........................13.75
12.........18..........450.........................15
13.........14..........350.........................16.25
14.........10..........250.........................17.5
15..........5...........125.........................18.75
16..........2.3.........57.5(probably wasted)20
17+........2.3.........57.5(wasted)............21.25

I have also measured temperature of the motor. Maximum was 46c (115f)

My only problem so far has been the loss of bottom gear due to placing an extra washer on the cassette side of the axel

Oh yes, and I was overtaken by a lycra clad roadster yesterday. I was doing 18 and he was doing 20.

But I comfort myself with the thought that his heartrate was probably >140 and mine was <120.