Consider that we are interested only in the speed and relative position accuracy of GPS receivers (i.e. their absolute try position on earth is NOT important). These receivers are all in a 90m x 90m area. In this case, does it really matter whether we use DGPS or not? For relative position/location accuracy, I think DGPS won't help much because the errors it corrects affect the receivers in a similar fashion (assuming that they see the same satellites, which is a reasonable assumption?) Does DGPS help improve speed accuracy (assuming Doppler based speed)?
DGPS or RTK provided corrections that behave like a translation for absolute GPS measurements, so for relative measurements and speed over small areas. absolute GPS device seems to be enough.