Expected is expected for that year group and is clearly based on abilities of the eldest
No, it's based on the expected ability - although it's certainly going to be easier for those who have more time to reach the expected ability.
It's not like the sept 7/8 yo are sat there thinking they must be thick if they aren't top is it!?
I certainly have heard of this - and heard of kids who have entered with the advantage of age (and an academic parent/nursery environment) losing confidence as they're no longer special for being best at reading etc.
It's so individual on the kid, population wise there are clear differences, but given that those most able to access deferring tend to be privileged, so likely to be higher attaining anyway it's not guaranteed that deferring is best.
Maturity and mental and physical capability is what will provide an easy transition to school, if you have any doubts on those, then deferring might be right, if you don't, then I think it's not worth it, certainly with DD, deferring would not have been a positive thing, and if anything I am glad she is summer born.