I didnt know that the etrex units had GLONASS. I personally have seen the difference that switching the non h versions to a h version using the star chips if i remember correctly was a huge leap in performance.
I have seen that the new edge 810 has a glonass built in and i was wondering how much of difference it could make as ive heard in smart phones it can increase accuracy to about 1-3ft. Im currently running a edge 800 and get about 2-3m in my city but still have some tracking issues while in the car with the unit.