Mockups are one of the best things that happen in here and will always be welcome. Having said that, here are my thoughts on the stormie voices in ESB:

I think ESB stands out from the other 2 OT films for a number of reasons, one of which is the increased dramatic tension our heroes are placed under during the events portrayed in the film. ESB is a character film above all else....and way more so than IV or VI. As such I think the focus pulls inward...from gazing out at this strange new universe and all its marvels (as in the cantina in IV), to a more intimate, "close-up" view of our main characters. We really get to know them on a deep personal level and have less time for the "look it's a space alien" or "hey, a new spaceship...let's see what it does" kind of moments. I think the storm troopers (who Han and the gang spend the entirety of IV running away from) are given a more secondary/background role in ESB....in favour of giving Vader more screen time, and making HIM the threat. I think the story of ESB held up so well that stormie voices were probably never needed as it would detract attention from what's going on in the foreground.

I DID like a couple of the additions (though it would be nice to hear them with original dialogue instead of recycled) (and yes, I know you only used what you had on hand).....the "yes sir" from the snow trooper being one of them.........I don't, however, have any need or desire to hear them say something everytime we see them on screen.......in fact many of the moments where you chose to put in a voice was actually taking away from the dramatic tension that was being built up in the scene (Han about to be tortured is a good example of this). BUT I'd say that your overall premise IS correct....they DO feel a bit more menacing when we know that they're thinking and voicing thoughts instead of just running around mute.