Yes, you should see less road vibration. It won’t eliminate it though. Yes, his higher weight (and thus tire pressure) would probably mean that he’s feeling more vibration.
The CL’s being designed around 26mm vs 28mm won’t really affect the vibration aspect but will have a larger impact on aero. So you’ll probably see lower rolling resistance and a more comfortable ride but you may be sacrificing some of the aero benefits. But those wheels will have no issues with mounting 28s on them.