I don't believe that a Ka6 will have any problem with a 6" chimney.. I agree with Mike on the summer scenario, a 6" will pull a more reliable draft in warm weather.
The only time I see where maximum chimney capacity is needed is when the stoker is running with a full bed of coal.. which would be when it is under full load, which is when the weather is really cold,, Cold weather provides a greater temp differential for the chimney and you have more draft..
I know they are different animals, but if an AA or AHS 130 can work with a 5" flue and the 260,000 BTU version work with a 6" flue,, then I don't see the 6" a limiting or safety factor for a Ka6 with much lower BTU output [hot exhaust gas output]..
And yes the AA and AHS use induced draft but in my instalation, I see a negative draft all the time when the combustion fan is running,, and this is with an AA260 with 6" flue pipe. My chimney is only 18' high..
I'm not sure which state would say it is against code to reduce a flue size... if it always pulls a negative draft in the combustion chamber then the chimney draft is adequate..