Humm wait a minute.. there is a: Tamron 70 - 300 , however not 2.8 ! it is a f4-5.6 or Tamron 70 - 200 f 2.8
One link about some examples of a 70 300 on a 7D : http://www.photonews.ca/?p=103 http://forums.dpreview.com/forums/read.asp?forum=1029&message=38578347
Tests of the 70-200 on 7D http://forums.dpreview.com/forums/read.asp?forum=1019&message=34064759 http://www.flickr.com/groups/tamron-usa/discuss/72157627872514314/
happy reading, cheers, Cj
Tell your friend he don't know what he's talking about. Canon 600D and 7D has the same DIGIC 4 sensor, so you would get the exact same picture quality from both cameras using a Tamron lense. The difference of these two cameras are design, features and build quality.
Anyways.. Have you considered a 18-200mm lense? 300mm is really tricky to hold steady without a tripod, and a 70mm zoom out limit is kinda inconvenient if you suddenly want to take pictures of a motive close to you. If you instead go for a 18-200mm, you can take pics of close motives in one moment, and far away motives in the other without having to change your lense all the time :-)
i can tell for sure that canon 600D is not a good one , even less better than canon 550D , of course 7D is better than 600 D , with 300 you have more zoom distance , so 70-300 will be better than 70-200, rather take f2.8 instead of f4 , you have better Aperture range . PS: some lens doesn't have manual focus , be attention ... ... nomarlly u need three lens if you want to really enjoy the result