Fill Flash Challenged?

+1 to GHwell's comment on FEC (not EC) but FEC can be set from the camera body.

But if you are seeing over-exposure, check the following: in bright ambient conditions, Av is probably chosing a SS above max sync speed. Then when you switch on your Speedlite, if HSS is not set, the SS will be forced back down to the max sync speed - ensuring an over-exposed image. (if HSS is set, max available flash output wil drop c 2.5 stops - but there still may be enough available under ETTL II). (Can a 270 Ex II do HSS - I'm not sure?)

If a speedlite is ON and recycled, on most bodies, the asterisk button performs a central spot pre-flash meter and lock (default is 16 secs).