If heavy cavalry could manage to remain saddled whilst driving a 12' kontos through someones' mail clad chest whilst charging them at near full tilt {which might be slow for the heaviest Katatank , but most kontos weilding cavalry seems to have been lighter and swifter} then I cannot percieve any reason they couldn't manage to maintain a firm purchase on their horse whilst imparting considerable energy to their javlins at a modest gallop .
The Numidians certainly seemed to be capable of this .

If there was any practical differance in distance Cavalry Vs. Infantry could hurl javlins , it would likely be negligable provided the Cavalry had room to make a short run up . Think of a circular approach to the toss similar to how a highjumper {human on foot , not equestrian} performs like this halfarsed diagram where the curve is the path the rider takes , the lines is the javilns flightpath and nought is the target )----0