The amount of transferring pax from CX to other One World flights will be the square root of zero - think about it. As a previous poster has pointed out both Qatar and Finnair both use T1.
It makes no difference which terminal CX use.....as long as it's not T3.
Imagine 300 bleary-eyed pax de-planing at 06.00 and ending up at the toytown Border post they have in T3. Tempers will fray, curses will fly and vows will be made never to use MAN or CX again. I've heard it all.
The last time I got caught, I too vowed to avoid T3 airlines and by and large I've managed to do that as a conscious travel policy.