With level 9 fanat and a beast giving 40% ias and created in a Zerker Axe you only need like 30 ias more from items to get max zeal rates. So you don't loose any IAS factors. As for Damage, the level 9 Fanat is like 130ish ED and a level like 25 Concentration is like 450ish ED. A level 25+ Fanat is like 560%+ ED and gives you 10% AR bonus per level. You can make a intresting build with beast but over all the normal faant build is much more effective for damage, attack ratting, and weapon options.