You know, if you're looking into house ruling this feat into usefulness, the obvious first step is to address that conflict.
For one thing, I do agree the requirements are pretty high, I'd drop the requirements so a level 1 can meet them. Honestly, at level 1 your average THF deals 2d6+6 for one hit kills anyway so why nix +1d4 on top of probably 1d6+2 until level 3 or 6, or even 9 which is the as-is? Then lower the cost of use. Like maybe during morning prep you can set aside one spell slot to use this ability once per encounter. It's still limited and pretty underpowered in the late game, but for low levels it's a nice quick and easy damage boost that thermionically feels gishy and a little Paladin like. Even as a side effect, you would be better set up to handle the ultimate gishy gish, the Arcane Swordsage, as they can normally recast as many spells as they want. It makes the really the only class that could make this feat useful but the change would knock a spell/maneuver out of their daily list. Which is something else to consider as well. Food for thought though.