I hope something from the Xpert line fits.
The challenge we're running into is the distance between mounting tabs. I've attached an image showing how the Futaba's fall short by 4mm total... other brands as well. The mount hole spacing should be ~42.5mm. When they fall short it's up to the end user to accommodate and adapt, which is what a lot of guys have done on other helis.
However, Synery can only recommend servos that are a direct fit without requiring custom adapters or large washers. So far the confirmed list is very small: MKS, BK and Savox/Align.