Hi Lowley,
Do you know how your school splits the classes? If it is by age, there is nothing you can do, your DD will end up in the mixed class anyway. If it is by ability, then I am sure they will look at ability at the END of reception, rather than how long a child was part-time or some such. Equally if they look at 'maturity'.
So how do you increase the chances that your child reaches her potential best maturity/ability by the end of reception?
If you are certain that she learns best when well rested and you think full time will be too much for her/cause her to be too tired to learn properly, then stick to your guns. Don't believe that 'maximising time spent in school' is more conducive to optimal learning outcomes than 'being well rested and ready to learn at the times when she is in school'. School is highly inefficient for learning (from an individual perspective) - the gains from actually being able to learn when in school, due to being fresh and rested, definitely outweigh anything extra she might be able to learn in the extra time at school if she went full time but was constantly tired and at her limits.
The actual learning in reception could probably be achieved with half an hour one-to-one per day and lots of self-directed play. We found with our August birthday boy that learning slowed right down when he started school as we had no time for doing anything at all one-to-one anymore (due to him being too tired after a full day at school where he barely learned anything). Academically it would have been much better for him to go part-time. (We had other, compelling reasons to go full time but I regret - more so in hindsight - that we didn't really have the choice.)
Also you cannot make a child more 'mature' by putting them into a situation that goes beyond their current maturity just like you don't teach a child to swim by throwing them into the deep end from the start. If you want to ensure that your child develops smoothly (e.g. becomes more confident etc) then your child needs to be in situations that are appropriate to her current maturity, with a little bit of challenge for growth. Again that would speak for starting part-time if you feel she is not quite ready for full-time yet.
Our boy wasn't too shy to ask to go to the toilet but was struggling with spoken and unspoken rules in the new environment, when to interpret things literally and when not. Teacher said 'everyone that needs the toilet go now, we're having carpet time afterward and I don't want disruptions then'. DS didn't need the toilet so didn't go. Half way through carpet time he did need to go but thought he was no longer allowed, so wet himself (thus causing even more disruption obviously). This happened several times and there was little we could do about it from afar - until we instructed DS to go to the toilet EVERY breaktime, whether he needed to or not, and before lunch too (he had several times wet himself because he tought if he left during lunch his food would be cleared away/because lunch supervisors didn't let children leave before they had 'finished' and didn't listen to explanations/because he thought it wasn't allowed/because he worried another child would eat his pudding).
So for all these reasons, if you want to maximise your DD's learning, and therewith her chances of going into the Y1 class next year rather than the mixed class, send her to school part time if you feel that that would be best for her (and it sounds like you do) because what's best for her now will help her learn better and progress more.
And yes, my SB did well too and continues to - and yours might too. Statistics are just that, statistics - they say nothing about individual children.
(The memory thing is fantastic! My DS was like that too. I think it is a skill we lose as we learn to read and write and thus can record things for easily re-accessing later; we don't rely on, and don't practise, our memorisation skills and thus lose them.)