I'm a bit confused. Were you using the 100-300 on one of your bodies or someone else's 7D? It sounds like you're comparing one lens and camera with a second lens and camera, not two lenses on the same body, which reduces the variables. Also, you may have experienced individual variations between the lenses, in addition to or instead of systemic lack of sharpness for a given lens design. And then you're asking for help on whether to buy a TC or a different body for yet another lens selection.
Are you contemplating replacing one of your existing bodies with the 7D, should you go that route? Otherwise there's a significant cost difference between the two options, and you'll have a third body to carry around. But you will have f/2.8 across the board. Between the 7D's high ISO performance and its higher burst rate than your existing gear, it'll also extend your capabilities. Replacing the 1Ds2 with it you'll also get a real boost in enlargeability from the added pixel density.