Hi Mike, your air cadet time will stand you in good stead with many organisations (not just APEM) both in and outside aviation, it all depends on the individual in charge of recruitment but faced with a large pile of CVs all from people with PPL and 100hrs you need to make yours stand out from the others and cadet membership will definitely do that for you.
Most teens have not done an awful lot with their time other than A levels and flying training so to have been a member of any of the military cadet organisations, to have played sport at a reasonable level (county level etc) or maybe to have travelled with a rucksack and a tent for a few months will get your CV noticed and pushed to the top of the pile.
The subject of military cadet forces was discussed on a thread some months ago and 2 contributors who were both involved in recruitment for airlines stated that membership of a military cadet force would get a CV through the first sift and on top of the pile for interview and assessment.
If/ when you get to interview the cadet force time will give you something to discuss with the interviewer and when they ask the dreaded ‘tell me about a time when…..’ questions you will have real life examples to regale them with.
As regards enthusiasm, you can be fairly sure that all applicants will claim to be enthusiastic about the job.
Good luck with the remainder of your flying training and A levels etc and if APEM do take another cadet later then I wish you well with your application.