What most people fail to realise is that you could have the fastest AF module that will ever be produced, but it would count for nought in the greater scheme of things. You see, an AF system is just that, a system, which is composed of various sub-systems of which the AF module is just one. The other major component would be the lens control and focus drive mechanism. I believe the AF-D system requires the lens to be capable of reacting and focusing fast enough and accurate enough to keep up with the A99's focusing system. Hence, the in-lens control electronics and the motor drive mechanism needs to be upgraded so that the results of an ultra fast and accurate focusing system can be translated in to real and tangible results, as opposed to theorectical on paper results. It may be the case that legacy glass may perform a little faster but it won't be anything to write home about. In any event, legacy glass won't be any slower, but I don't think - and this complete speculation, that Sony will add legacy glass to the list of AF-D useable lenses precisely because they don't have the electronics or motor drive mechanism (SSM) necessary to take advantage of AF-D speed and accuracy. Legacy glass can't react and move any faster than it already can/does. That's my theory until proven otherwise.