Hi Dave

I had a similar situation and swapped the non IS for the IS version. To my eyes and as seen on my monitor, I experienced a noticeable improvement in IQ with the IS lens on both crop (7D) and FF (5D2).

I also used the 1.4 (Kenko) TC with both lenses and found the IQ impact to be acceptable, although somewhat softer to be sure.

The IS version is just a sweeter lens to use in all circumstances.

Regards

Jimmy