For winter I use my old do it all LX equipped (discs running gear) Mtb HT with slicks for Tarmac, canal paths and tempted to go further until I can use my road bike when the weather is drier. Except I’m sitting upright and drag hinders top speed.

Would it make much difference fitting 27.5″ wheels?

I sat on a CX today. Nice fit on the hoods or bar but cramp/aero on the drops – weird as the top tube was 55cm and I use a 57cm road bike.

Would it make much difference fitting 27.5″ wheels?

The wheels won’t change the position but you knew that. But like you I’m tempted by better rolling(?) bigger wheels. Although the fast rolling road 27.5 tyre selection isn’t huge

It will make negligible difference.

Rolling resistance is lessened with wider tyres, it’s the aerodynamics which will be slowing you down.

I thought aero didn’t come into play unless doing 20mph or more?

I think I will just go the CX route and sell my mtb 26″ hybrid and XC bike as the CX fits both categories for me.

Think I need 700c wheels and drops. Sitting almost up right on the hybrid is acting like a sail.

Thanks for the advice.

You could try fitting drops or bar ends to your hybrid and see if that helps?