You are confusing Marathons with Marathon Plusses. Marathons are not "heavy suckers" nor is their rolling resistance notably high. BA's claim a low rolling resistance at low inflation pressures but I have to think that the rolling resistance is lower at the higher inflation pressures? I run mine near the top of the pressure range anyway. Inflation pressure more than tire size is what determines rider comfort. You can find, if you look, 32(!!)mm tires that will go down to 50psi and offer a very comfortable ride. Trust me, there are ~400lb riders rocking 25mm slicks @ 100 psi in relative safety if not comfort. There are no rules. I personally would not even consider using another manufacturers fork in a bike to get tire clearance. I would be concerned about the effect the change in head tube angle would have on vehicle trail and the resulting effects on handling and stability. Negligible probably but still... the cost wouldn't be negligible. FWIW.

H