Which ND Filter ?

I use a 70-200mm 2.8 and shoot motorcycles at about 1/400 or 1/500 and F9-F11 during the bright daylight. I would like to get better depth of field by getting the lens down to F2.8 if possible, but any lower F-stop would be nice.

I would go for a 3 stop filter right? To go from F11 -> F8 -> F5.6 -> F4

I read somewhere that the Tiffen filters seem to be most color neutral. Any opinions either way on that one?

Also, does using a larger aperature increase the camera's focusing abilities/speed? My D70 seems to miss alot and is kinda slow.

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-brian