I don't have a specific document on hand but I believe all possibilities regarding minor relocations of Newmarket GO have been considered, and they've found that there is no viable option for double-tracking/platforming it while in proximity to Davis due to a number of constraints as revealed by studies. Mulock and EG are the more workable options.
Also, it's largely a moot point as there is no need for a double-tracked/platformed station at Davis because there is no intention of having service every 15 minutes there, only hourly. The 15-minute service is currently slated to run only up to Aurora; I believe a part of the basis for Mulock station is that it would be doubled and support an extension of the 15-minute service that far, but I think Metrolinx considers even that to be a stretch regarding time+money required vs ridership. Again, hourly service can run just fine north of Aurora with fairly minimal construction, and would be a massive improvement from the current peak-only service.