Why do bolts have to fly faster than arrows?
I see many crossbowmen (including mounted ones) who are doing fine, more than (horse) archers actually.
Also keep in mind that the archer will have to spend extra skillpoints on strength and PD. A PD 5 strongbow horsearcher will still do less damage than the light crossbow and that already makes it hard to get to 4 horse archery without gimping your character otherwise. The horse crossbowman can easily stop at str 12 and call it a day - skills go in PS, athletics, etc. instead.
The wpf suggestion is good, but I think there are some hard limits how accurate weapons can be (which cannot be overcome without introducing potentially exploitable mechanics like archery formula before the patch).
edit: From a realism standpoint. Let's recap that projectile speed = squareroot(2*draw strength*draw length/projectile mass) (*efficiency factor).
Now, since bows and light crossbows are drawn the same way essentially, the draw strength of the crossbowman can't be higher than that of a bowman who regularily trains (skillpoints), but the length is shorter, so realistically a hand drawn crossbow would always be weaker (either less damage or less speed) than a hand drawn bow. Only crossbows with special loading mechanisms could be more powerful than bows.