Oh, Missed that, yes a 340mm f5 is not bad at all as the minimum focus distance should remain the same, right?

A good photographer should intuitively know his equipment, that doesn't happen overnight. If someone changed my camera I'd have to keep reminding myself where certain setting are. EV Comp, Sensitivity, Bracketing, Manual WB, Shooting Mode and more. Yes it is doable, but not with the same level of competence. It would be so easy to miss a once in a lifetime shot while fumbling around with settings on an unknown device.

Nothing wrong with the 30 f1.4 and 28 f2.8. If you want shallow dreamy DoF you're a bit stuck with the other lenses as they are slow and not sharp at their wider settings.

Not saying it can't be done or the results would be terrible, just saying it would be better if he used his own equipment.

Does he have any fast Canon glass? I guess if his kit is really lacking then he'd be better off with your stuff but then this will run round in circles as we are back to unfamiliarity, fumbling and missing shots.

Carry less lenses?

I think the 200 is a little short as well, especially as its magnification is also less than the 300mm. Even at minimum focus distance on the 300mm small birds still don't fill the frame. They'd require even more cropping with the 200mm.

How big?

I'm looking to change my bag as well and am almost set on the Tamrac 3447 link.

I have tested it for fit and can put in:

K10D + DA*16-50 mounted
K10D + DA*50-135 mounted
2x Metz 58
4x Cactus V5
Spare batteries, cards, remote releases
Laptop
Tamron 90mm Di
and still have space for a small prime (maybe two or three if I had the Ltds).

I'd be worried about using equipment I was unfamiliar with as different cameras are not always straight forward to swap between.

Also given you're equipment list he may be better off with the older Canon DSLR he has. I'd take fast lenses over a high sensitivity camera any day especially shooting a wedding. You just won't get the same quality otherwise.

I do carry my DA 18-55 and 50-200 with me for weddings but they are there as a last resort backup. I'd use my A28, A50 and Tamron 90 before I even looked at the kit zooms.

At my wedding we had a pro using a 5D MKii and a 40D is equal measure. Can't tell the difference between them until I zoom to 100% (got the files as he is my wife's cousin). I really wouldn't have cared had all the photos been an a 40D or 350D, the camera is only as good as the person standing behind it. I wouldn't want to confuse someone by changing their equipment.

Unusual to use a 200mm equivalent telephone for landscapes, but not unheard of.

It is a little, bit the flexibility of automatic exposure when on the move means sometimes you have no choice. I've not tried A mode with bounce though so that could be an improvement when using something like the Metz 58.

Lou would probably say you just need to use it more. She shoots almost exclusively with her 90mm Tamron and much prefers prime lenses.

Seeing these I bet you are glad you didn't get the 60-250. Amazing variety of captures.