I fitted the electric fan in a bid to (as always) reduce fuel consumption, increase warm up time, etc. It was quite a faff, so once it was on I left it there and drove around with the viscous in the boot! I don't honestly think it made much difference, so to anyone whose viscous is working properly, i'd say, leave well alone. But I have read of some whose viscous has stopped working, and it is not easy to drive around with all the electric fan stuff in the boot!
As for stats, I have heard of real problems with trying to insert them into tubes and then seal them, or put them in between the fins on a rad. I even heard that the best place to fit one is into the outlet pipe junction rather than the inlet, or was it vice versa? (This was a long time ago!) So a T with the capillary sensor fitted would appear to be best if you have room!
So it appears the viscous has a lot going for it!
Anyway, this is largely off topic as the OP is talking AC fan, not coolant. so sorry for that. Interesting all the same.
Wouldn't touch D3 or later, absolutely not!