When I used DOT 5 it did not cause higher pedal pressure only a mushy feel because it is slightly more compressible than DOT 3 or 4. With the mechanical advantage at the pedal it becomes very noticeable.
Higher pedal pressure could be caused by the coagulation of DOT 3/4 and DOT 5 if the systems were no properly diassembled and cleaned with alcohol to remove the last traces of the incompatible fluid.
Despite what they teach you in school liquids are compressible, but the amount is miniscule compared to gases.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo