People periodically ask if the model XXXX(X) servos is adequate for use as a throttle servo. Chances are, the answer is yes, no matter what the Xs happen to be. Here's my thinking:

You don't need much torque.

The throttle servo is only going to be working the carburetor. Grab the throttle arm with your fingers and give it a wiggle. It's probably the most free-moving control on the helicopter, unless you've done a stellar job setting up your tail rotor linkages. Torque is no big deal.

You don't need much speed

There's a common (mis)conception that the throttle servo needs to be at least as fast as the collective servo. Some people go as far as to suggest that the throttle should be faster than the collective so that it will 'lead' the collective.

These ideas don't hold up to close scrutiny. The collective will only 'outrun' the throttle servo if you're slamming the stick up and down faster than the throttle servo can keep up. The throttle servo will only 'lead' the collective if you're slamming the stick up and down faster than the collective servo can keep up. In normal flying, the collective is moves smoothly and gradually from top to bottom, at a speed that's pretty much in sync with the helicopter's roll rate. Neither servo is being asked to move faster than it's capable of moving, so both servos are always right where they need to be.

The servo will be under great vibration

Possibly, but not more than any other servo in the helicopter. The only thing that would cause the throttle servo to get more vibration than the rest of the servos is the throttle linkage. If you've got ball joints at each end of the throttle linkage, this is probably nothing to worry about. Side-to-side and up-down movements of the engine will just cause the ball joints to change their angles, they won't be transmitted to the servo. Any engine movements in a direction parallel to the linkage itself will be soaked up by the rotation of the throttle barrel. The remaining vibrations will be coming from the chassis itself - just like the rest of the servos.

So, in the end, it doesn't make much difference. My Concept has had a Futaba 3001 throttle servo for about four years. I just bought my Bergen Intrepid 3D, and bought another 3001 for its throttle. My Futura has the nicest throttle servo - same torque as the 3001, but a speed of .17 instead of .22 - and I think I spent more than I should have!