Is this the beast you use or plan to use?
http://www.hobbywing.com/product_show.asp?id=284

If so then it delivers 6V to the receiver so the WR-4401 servo would be a good choice.
The WR-4401 delivers the following speed and torque at 6V
[email protected]: .085 sec/60
[email protected]: 18.40 kg-cm (255.39 oz-in)

That should be pretty decent as standard. Or you could choose to bypass the ESC's BEC and use an external BEC like Castle's 10A SBEC where you can set the voltage to pretty much what ever you desire within a set range of course.
I have a Hong Nor Nexx8 i'm planning to get working when I get around to it and I have put a WR-4401 in that one and chosen to use a Castle 10A SBEC to bypass the Mamba Monster 2 ESC as I have heard the internal BEC in the Mambas are very frail and so they burn up when put under too much strain.
I have set the voltage to the receiver to 6.6V so the WR-4401 delivers
[email protected]: .077 sec/60
[email protected]: 20.24 kg-cm (280.93 oz-in)

I hope all my rambelings can help you.