Those removal rates are not accurate.   The number you cite comes from an Iowa study that included 100% of the stover, I think at physiologic maturity and not at typical harvest date.   The lower 1/3 of the stalk contains the bulk of the remaining macronutrients (mass x concentraion).  You would not be taking that portion of the stalk in any residue harvest scheme.  The later in the season you harvest and the more rains since plant dry down, the further the reduction the macronutrient contents. 

see these publications:

http://www.ars.usda.gov/SP2UserFiles/Place/66570000/Manuscripts/2010/Man843.pdf

http://poet.com/media/Stover-Harvest-Report-May-2011-Version-3_copy1.doc

Keep in mind that most biomass plants will need to get rid of ashes that maybe made available cheap/free to contracted farmers.   I would expect most of the potassium to be availabe in the ash.