MU30 FLC OVERSHOOTS ASSIGNED ALT ON DSCNT WITHIN ZKC AIRSPACE.

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ON DSCNT INTO COU; WE WERE WITH ZKC AT 15-20 DME. WE WERE ISSUED A DSCNT TO 9000 FT MSL; ALSO A TA 12 O'CLOCK POS; 10 MI AT 8500 FT MSL. SHORTLY AFTER; WE WERE ISSUED A CLRNC TO 8000 FT MSL. NO FURTHER TA'S. GOING THROUGH 7600 FT MSL; ABOUT 10 DME; COPLT RESTATED 8000 FT MSL CLRNC. CAPT THEN IMMEDIATELY CORRECTED ALT BACK TO 8000 FT MSL. WHEN CORRECTING THROUGH 7800 FT MSL; ATC INQUIRED PRESENT ALT. COPLT STATED 7800 FT MSL. SHORTLY AFTER; WE WERE HANDED OFF TO COLUMBIA APCH WHERE WE WERE ISSUED A VISUAL APCH. FACTORS INVOLVED WOULD BE AS FOLLOWS: CAPT'S MISUNDERSTANDING IN DSCNT CLRNC. CAPT'S ANTICIPATION OF TWR SINCE WE WERE ENDING UP ABOUT 7000 FT AGL; 5-8 DME FROM ARPT. MISSED '300 FT TO GO' CALLOUT. ALSO; AT HIGHER AIRSPD; CAPT'S AND COPLT'S ALTIMETERS DIFFER AS MUCH AS 300 FT. SO WHEN COPLT SAW 7900 FT MSL; THE CAPT'S SIDE WAS ALREADY 7600 FT MSL. THIS DIFFERENCE IN ALTIMETERS IS CAUSED BY THE CAPT'S SIDE USING AN AIR DATA COMPUTER; AND THE COPLT'S SIDE NOT HAVING ONE. FATIGUE WAS ALSO A FACTOR DUE TO MULTIPLE LEGS THAT DAY IN MODERATE TURB; WINDSHEAR +-10-15 KTS; AND XWIND LNDG COMPONENTS 20-30 KTS.