The last few missions doing GTO boosts have had the ASDS in reasonably close to the same location in the ocean. The lower energy CRS-8 mission was further in shore, since it had the performance to come back that much closer to shore.
There is a fair bit of variability possible, but it does appear that there is little benefit in going much further east, since if there was, for the SES-9 (That crashed hard), and the other high performance missions they would have taken advantage of that.
There might be a limit on how far they want to tow an ASDS out to sea, manage it for a day or three and bring it back. It does take about 4-5 days to get to the current GTO landing spot. The support ships have been shown to have fuel supplies sufficient, but there might be a crew usage issue.
Additionally, missions to GEO like orbits have some options. GEO is harder than GTO both due to performance, but also due to restart and coast issues.. GTO requires that the upper stage/payload do more of the work to get to GEO than a direct injection into GEO. SpaceX usually does GTO not GEO launches, because their second stage cannot coast long enough on batteries and make it GEO, thus they quote GTO, in which there are several standards for delivery.
Depending on the orbit, the payload itself needs to expend more energy to get to full GEO, which either takes a long time (Electric thrusters) delaying entry into service by months, or else reduces its on orbit life span by using up the fuel to get there.