There is a dagger called TorGals Claw +3, which lowers Str by 3 for 60 seconds if the target fails the save (not sure if it is original BG2 or has been introduced by one of the mods I have installed).
Haer Dalis Chaos Blade lowers Dex by 2 for 12 seconds.
Kachicos Wakizashi (the one you mentioned that's sold by Joluv) drains 2 Wis for two rounds. This could indeed be interesting.
Well, my char (currently F22/M19/T26) hangs around in the slums, so a quick test is no problem. Fortunately I also sold the Chaos Blade to Hendak, stripped the blade from its former owner a long time ago. And I have the dagger in question with me, so let's test this:
1) I visit Joluv, buy
Kachicos Wakizashi and have a look at my stats. My base THACO is 0, with this weapon it's -6. Looks great. So I attack a well known evil dwarf (Str 18/77, Dex 15, Int 12, Wis 9), who happens to be standing in the corner of the inn. I use GWW of course. He saves vs. Death 9 times in two rounds, not a single point of Wis loss is indicated and he obviously dies from physical damage.
2) Reload, memorize
Shapechange, I turn into a
Mindflayer, use GWW and attack the dwarf again. My THACO is my base THACO, so it's 0. After three hits and 21 points of physical damage, the dwarf goes down. Obviously, 4 or 5 Int are drained per hit. Since I have tactics installed, it's likely 5.
As for the picture in the post above, there were two more "Devour Brain" messages below the line that indicates death - the indication of events is not completely sequential and death is sometimes indicated before the damage that finally caused it. The same counts for spell animations, which sometimes happen after the caster is already toast.
3) Reload, buy the
Chaos Blade back, go GWW and attack the dwarf. THACO is -5 and I kill the dwarf with 9 hits in two rounds by death from physical damage (at least no Dex loss is indicated and the number of hits is same as in 1, dealing roughly 140 hp of damage).
4) Reload, memorize
Black Blade of Disaster and turn on GWW. Well, the dwarf makes all his saves again but goes down from damage already in the first round this time (which I had expected, due to specialization in long swords my THACO is -13 with BBoD).
Speaking of BBoD, a little off topic: BBoD combined with Imp Haste and Fireshield (Red) is the ultimate troll killer if they bug you in large groups that include higher trolls (spectral, spirit, giant). This is not only a very fast method, you'll likely leave the battlefield completely healed. Altough some may consider it a little overpowered ...
5) Reload, use
TorGals Claw +3, which I have stored in my bag of holding, with GWW. THACO is -5 and again, 139 hp of physical damage and a bunch of saves vs. death later the dwarf goes down.
Conclusion: Out of 3 stat draining weapons, one potentially level draining weapon (BBoD) and the mindflayer attack, only the mindflayer attack actually killed the target at clearly positive hp, indicating a death by successful (brain) drain.