Here are the data from such an experiment. Speed discrimination thresholds normalized to that for a single grating and averaged across 4 observers are plotted as a function of total stimulus area. The solid symbols show thresholds for the case when the number of gratings was increased. Thresholds decreased with increasing grating number. However, this decrease was not due to increased stimulus area, as increasing the area of a single grating as illustrated did not show a similar improvement.
Thresholds remain unchanged with increasing grating area.
At this point, I would like to note that we too found the lack of the classical area summation surprising. We observed a lack of summation in the range of suprathreshold contrasts that we used, but at contrasts close to detection threshold, increasing the area of a single grating gave the classical probability summation result. To explain this rather intriguing lack of an area effect at suprathreshold contrasts, we propose the following hypothesis for speed discrimination. The decrease in threshold in the case of multiple gratings is due to each grating providing an independent sample of speed, while one large grating gives only a single estimate. We measured speed discrimination in the fusion case where we merged multiple gratings, until they became a single stimulus.