I haven't noticed a 3000K shift in color with any of my B&W filters. Even my old cheapo colkin ND filters aren't that terrible, but then again I'm using the Pro-glass LEE and Schneider 4X4 & 4X6 GND & ND square filters.
You might want to look into a square system, Its fantastic.
In hindsight I should have went the big stopper route, though to be fair, the B+W was hardly a budget option.
I'm interested in the 3.0 filters you've used made by cokin, can't find these anywhere?
Had a look for the Schneider 3.0 filters as well, having bother finding them, other than as being sold under the sister B+W brand. I could find the Schneider square filters at up to 1.8 ND, but not as high as 3.0.
Just interested in the comparison you've made between the colour shift on my B+W 3.0 (N.B. 1000 NDx, 10 stop, not 3 stop) and the similar cokin and schneider filters you've used.
Can you give me a link to the Cokin 3.0 ND (1000x, 10 stop) filter? If they are as cast free as you say then I might end up getting one.