I have never ridden the Bianchi with the Fulcrum's on, but ridden both bikes with the Zonda's on. The upgrade of wheels on the Defy is almost as much as the upgrade from Defy to Bianchi. Therefore, I would have to agree with
@Fab Foodie to some degree.
My mate reckons that a mate of his bought a fair prices Scott in the sale and swapped wheels for carbon aero jobbies costing around £800. His total spend equates closely to mine. He has ridden my Bianchi too and reckons that his mates Scott is nowhere near as nice or as fast as mine, despite the wheels being considerably more costly than mine.
Therefore, it must all be relative I guess. No point whacking £800 wheels on a mid range bike and expecting to see a huge gain compared to a better framed bike with decent but lower specced wheels.
I reckon that the Zonda's hit the sweet spot on mid range bikes perfectly!