Hi Essyou35

The JR 8711 or evan an 8511 for the elevator is ideal, there are very few servos better than the JR range of servos.

The Viper will "Wiggel" a littel in the air a bit when its bumpy but usaly tracks nicely most of the time, on the ground its straight as an arrow
I cant see that a gyro would hurt but its not realy nessersery, I will pop one in for the next outing a see how it reacts and let you know. 

The Hitec 645 however is a analog 7-9kg servo, I have used them often on nose wheel steering as there inexpensive with strong gears.
Unfortunately there holding power is not all that good, in fact plug one in and then physically try to push the servo arm away from neutral.
You will be surprised how far and easily it will move and then bounce back when released, an ideal set up for flutter when flying at speed.

This tends to be a trait of non digital servos but quite noticeable on the 645, perhaps OK for smaller slower flying airframes but not the best servo for a fast jet.

On my Viper I used JR 8421 on the flying surfaces but any good digital servo of around 8kg will do but do the "bounce" test to be sure of it holding power as that's what counts.

Ive found the holding power of the 8421 is very good, its metal output gear gives strength were needed on the spline and the remaining plastic gears result in low backlash.
The JR 8411 is exactly the same servo but are fully metal geared, after a "lot" of use they can develop some backlash but it takes a lot of flying!.

 

You don't need a super high tork Digi servo and defiantly not a silly fast one but do set up the servo arm to horn linkages to use all the available servo movement.

It would be silly using 10Kg servos at only 50% travel rate to archive full control throw at the flying surface as surly its then only a 5Kg servo?

One servo I avoid on an elevator is the Hitec HS-7965MG Digi servo, although its rated at 8-10Kg Ive found its holding power alarmingly bad.

See the vid at the bottom here of one I removed from the elevator linkage of a JL Hawk bought built that I was asked to check out before it got a chance to fly!!
 

Here is a vid of a HS 645 servo on both a fully charged 4 cell nims pack and on a reg at 5.3v, its not realy much better on 6v, I would not recommend use of this servo on a primary control surface?