The derailleur isn't the question. All 8 or 9 speeds will work due to the overall width of the cassette being equal, the derailleur "throw" will be the same.
I've just always thought (or led to believe) that the amount of cable pull at the shifter corresponds to the spacing of the cogs in the cassette. So, if you run an 8-speed cassette, you NEED 8-speed shifters, and conversly 9 spd cassettes you need a 9-speed shifter.
What Kona is saying is that you can use the new X.7 9 spd shifters WITH an 8 speed cassette. True Kona?
I didn't think this was possible, but I learn something new everyday.
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