70-200mm f2.8L + 1.4x extender vs. 100-300mm f4.5-5.6 wide open?

Bit of a long title there! From what I've been reading here, adding a decent 1.4x extender to the 70-200 f2.8 (non IS) does not seem to affect image quality too much.

So I'm assuming that wide open at 280mm f4 (with the 1.4x extender) the 70-200 image quality should be at least equal than the 100-300m at f5.6?

Basically I have the 100-300 now, and am wondering if it's worth keeping if I get the 70-200 f2.8 + 1.4x extender?

Cheers,
Alex.

the 100~300, I'd say the 70~200 w/TC will be as good.
Other lenses would beat it though.

Here's my Sigma 70~200 with sigma 1.4:
http://www.pbase.com/b​enjacobsen/sigma_14x_t​c
and the lens alone:
http://www.pbase.com/b​enjacobsen/sigma_70_20​0mm

Ben