No, still the same difficulty to ride. AFAIK the only time it makes a piece of equipment go up in requirement is for some weapons, it makes them "heavy" moving up the difficulty one. Horses don't do that though.
Also Alex your riding skill barely changes how fast you ride or how maneuverable you are. I can't really tell the difference between level 3 riding on a rouncey and level 6 riding on a rouncey.
I'm a lancer cav, but have 1h and shield WPF/skill for on foot, and for sieges (strategus mainly). I split my WPF 119 and 118 at level 30 (6 wm). I personally like having WPF for my heavy lance, I'd rather have it extended as quick as possible.