There are not that many reliable labs with the right machines, skills and access to new GPU chips, but some like Rossman in N.Y.City undoubtfully can do the job properly and might be worth the steep costs they would be charging. After all these are the last 15" non retina upgradable Macs out there and still pretty efficient besides GPU issues.
The only caveat here is to avoid people who would do "reballing" and kick boards in one kind of oven or the other in the end.
@danj