I actually got taken in by David Zeiser's good review of Tamron 28-300 VC (http://digitalprotalk.blogspot.com/2012/04/test-driving-new-tamron-28-300mm-vc.html) ... but now I'm thinking maybe he was paid to do that review
As I stated when this link was posted above, if you actually ignore his words and look at the images which he describes as 'tack sharp' it's pretty clear that they aren't.
could you post some 100% crops with your 7D and the 28-300L / 24-105 for comparison ?
Both of these lenses are on my wishlist - provided they give acceptable sharpness on my 7d.
Sorry, I picked up the 28-300L after I had the 5DII and I never even used the 28-300mm on the 7D. IMO, 28mm on APS-C is not wide enough for general purpose use (it's 45mm FF-equivalent, which is normal not wide). For APS-C users, I dont' recommend the 24-105L or 28-300L as walkaround lenses due to the lack of a wide AoV - the 17-55mm or 15-85mm are much better options. I did use the 24-105L on the 7D occasionally - I initially bought it for use mainly in inclement weather with the 7D, but I didn't put the lens to serious use until getting the 5DII.
I have compared the 24-105 and the 28-300 on the 5DII, and as I stated, they are pretty similar (both better at the wide end, a little worse at the long end, similar in terms of CA, etc.). Neither is a stellar lens in terms of IQ, but I'd say both are very good, and very convenient.