Since I will lose an f stop and autofocus, I'm wondering if I'm wasting my money in buying the Kenko Teleplus PRO 300 DGX 1.4x AF Teleconverter for my 400mm f/5.6 lens? I can shoot in bright light if that makes a difference. I would like the added reach but not at the cost of IQ. Talk me into it our out of it! Thanks.