Rode the Giro course again tonight, this time on a light-ish bike and wheel set (I don't have enough Drops to get the really light bikes yet), compared to the TT/808 (non-disc) setup I did on the first ride. Now it's not a completely fair comparison, because this time I deliberately took the flat section a lot easier to save energy for the climb, plus I was quite tired after work, but I was surprised by how much difference the two setups made.
Using the "official" ZwiftInsider segments:
Flat:
TT bike - 10:19 (220w avg)
Light bike - 10:56 (205w avg) - going to assume I'd have about halved the time difference if I'd have produced the same wattage
Hill:
TT bike - 14:27 (242w avg)
Light bike - 14:06 (241w avg) - sheer accident I got so close on the wattage between the two climbs!
Overall:
TT bike - 24:47 (233w avg)
Light bike - 25:02 (225w avg) - think the lighter bike might JUST have sneaked a win with the same effort over the entire stage
EDIT - although now having looked at the comparison between the two efforts on Strava, it seems that I finished a LOT faster tonight. I was 37 seconds down at the start of the climb, but a whole minute down when I reached the first chicane (just before the 15% section). I then pulled that back to 37 seconds again by the second chicane and down to just a 15 second gap at the finish line.