Speaking as the parent of a late August born DD and a mid May born DS, I am concerned about this.
Unless there are specific needs that have been identified by professionals associated with the child or family AND those professionals believe that the child must be educated out of his/her year group because of these needs, then I believe that children should be educated in the correct age group.
At present there is the potential of 12 months difference in age in a cohort if these proposals resulted in many parents of April - August children delaying entry, then you're just moving the problem to children born close to 31st March. More so, there will be parents who are not aware that they could delay entry, or they simply cannot afford to delay entry, for their late summer born DC and those children could conceivably be in a cohort with children up to 16 months older.
Secondly, when a parent has secured delayed entry, it is imperative that they secure it for the rest of the child's education. There are cases of children having to skip Year 7 and go straight from Year 6 to Year 8, there are also cases of families who have moved and the new LA have placed the children in their correct age group, meaning the children have, effectively, missed a year of education.
Then you have parents whose April - August born DC miss out on their first choice school. They could conceivably choose to reapply the following year. I cannot begin to imagine the knock-on effect this would have for schools in very oversubscribed areas, like London Boroughs and Bristol...
The current research on summer born children is based on children who have had 2 fewer months education. It is only recently that all children were able to start in September; therefore the results are biased. I believe - and this is only an opinion - we will see less of a summer born effect as those summer born children, who have had 3 full terms of reception, education go through the education system.
Honestly, I can only foresee problems with this.