Your question can't be answered the way you asked it. I think what you want to ask is "what's the difference in power needed to ride a MTB and road bike at a given speed" and "what's the power loss due to rolling resistance between CX tires and slicks."

A typical MTB CdA is about 0.4 m^2, a typical road bike CdA with rider in the drops is about 0.32 m^2. So at a given speed, the MTB will need about 25% more power to overcome aerodynamic drag.

Rolling resistance coefficient (on pavement) for a typical MTB is 0.006, for a road bike it's 0.003, so MTB knobbies eat about double the power in rolling resistance that road bike slicks do (plus a little more because the MTB is typically heavier). I'd guess CX tires are somewhere around 0.004, so 33% more loss that road slicks.

Now, in terms of power percentages, it's pretty complex, but it suffices to say that at 22 MPH a road bike rider needs on the order of 170W to overcome aerodynamic drag and another 20W for rolling resistance (there are drivetrain losses and other stuff, but it's not really a differentiator). The MTB at 22 MPH would need +20W for the tires and +40W for drag (+60W total to keep up with the road bike at 22 MPH). Pretty significant, but not as bad as some people think.

A CX bike with all-around cross tires is going to need about 7W more to maintain 22 MPH than it would with slicks. File treads would be better, mud tires worse. How much that's going to slow you down depends on how fast you want to ride.

The main reason not to use CX tires for road is that they wear really fast and they're not very puncture resistant (not a lot of broken glass to deal with on the CX course -- hopefully). If you think the "OEM" tires a bike comes with are carefully selected to produce a certain "feel", you're being idealistic. I think the industry insiders will confirm that OEM tire selection is all about availability and meeting a certain price-point.

Disclaimers: calculations make massive generalizations and are based on pretty old data (road bikes were less aero and MTBs all rode 26" tires). Data from Bicycling Science 3rd ed., Wilson, David G., and Jim Papadopoulos, MIT Press.