The rules are grey on the area regarding whether a feat can be used multiple times on the same spell. Some figure you can because the rules don't say you can't. Some figure you can't because the rule say you can.
This leaves the fallback everyone should be using: is it reasonable to allow someone to increase Dex by (1d4+1)x2.5 for basically the whole day as an 8th level spell reasonable? Now, what if this was stored on, say, a Staff of Buff Cubed (bull's strength, cat's grace, endurance; all empower x3; caster level 15; base cost 180K gp; cost to create 90K gp and 3600 XP) with 50 charges?