I have no experience with the Shadowtech line, but after reading that forum thread you linked to, I have a little doubt.
In T3, monsters are generally more resistant to Knockdown. If he used the 4* in T2, then it may have had knockdown. In T3, it may not.
(Example: Zombies in T3 can barely be knocked down.)
The original poster did not mention where he was, so I can't say if it's for sure or not.

Don't take my word for it, though. It's just a thought.

Actually, there are two cases I've found, at least with the magnus line, where knockdown stops working reliably:

- Last stratum of T3 (it should work fine in the first part of t3)
- Danger Rooms

Unfortunately, I haven't played with any of the alchemer lines, so I can't speak for this case.

I have an Umbra Driver and am very happy with it. I can test out the knockdown, but I really don't use it for knockdown which is what my Callahan is for. I always have it on me for gremlins and slimes.

I'll grab some footage from d16 and post it up for you.

Edit: http://youtu.be/OkooqN4eMmY

81 damage vs. 84 doesn't seem like a huge difference, but I'm not sure if that's the weapon scaling for this level, just the way the damage has increased period, or something different in our gear/UVs. I have no damage bonuses vs. slimes or other general damage increases on my gear. At least there's a video now so you can decide if you feel it's worth the upgrade. I appreciate the extra bullet ricochets, particularly in tightly packed groups of jellies.

The damage will very depending on heat, depth, monster type and sometimes by individual monster too (depending if they were taking from a different strata or have special defenses.) You would need to go to the same depth and kill the same monster to see the damage increase as heat progresses.

Given the above condition, if the Umbra Driver really doesn't increase in damage from Level 1 to 10 that sounds like a bug. Oh, also, be sure you are toward the end of the clockworks. Damage nerfing based on depth for 5* weapons prior to the last strata could affect the bonus you get from heat.