I have been running a 12 row since 2000,  I don't think you need the corn seive,  I never thought they cleaned as good as the regular seive.  Have you change to all foward bars in the rotarIJ  Or better yet put in a CDF rotar,  it is amazing the difference in capacity,  getting a cleaner sample,  and savings in fuel.  I think I used 10-15% less fuel both in corn and wheat.  As far as corn head goes,  the adjustable deck plant are really nice expecially like in my case I cut both a lot of dry land corn and irrigated corn.  I also like having lateral tilt on the combine if you cut over terraces or rough ground.  Just my thoughts  Kurt