In the past I have used 3421's then 3717HV's for higher voltage installs, especially on all my V-tail models. I just got two new servos. The JR MP31Swv and the JR NX3125. The MP31Swv is the fastest of the bunch. It is down to .08 seconds on 7.4 volts. It is a wide voltage servo so you don't have to worry about what battery pack you are using. It also has just a little more power than the 3717HV. The case size is the same as the 3421's.
The other interesting item is the NX3125. This is not quite as fast as the MP31Swv but it is an XBus servo. It's also more flexible in voltage range and it us XBus programmable to tweak the servo if you need to. What I'm really hoping for is to get the JR398 servo in a NX version and a receiver that just has maybe 4 XBus outputs to use in a sailplane.