Yes, I don't think requesting a statement is the way to go either (recognising that statement will not earn me any fans). Reduced writing speed at 25-50th percentile means that he is in the lower half of his age range, it's actually an average range score that between 25 and 50% of children his age will be getting, so I wouldn't really think that counts as a significant issue, if you think about it, statistically half to 1/4 of his class will have the same writing speed and 1/4 will be slower than him.
Working memory at 12th percentile is more significant but only just below the bulge of the statistical bell curve of normal distribution. Percentiles can be tricky because they are easy to understand in one way but in another way it's confusing. Basically between 15th and 65th percentile you get a massive group of children (66%) who all fall within average range and in terms of the scores between them it's not evenly spread, so for example between 1st and 4th centiles, you might have 6 marks difference, but only 10 marks or so difference between 15th and 85th percentiles, because it is rarer to be a very high or very low scorer compared with a middle range scorer. So 12th percentile is only a little below 15th percentile which is the lower end of the average range. It is something which will make aspects of learning in school harder for him and I would have said he should be on school action and have some adaptations around expectations for his memory load - lists, written instructions, colour coding of books, checks that he has recorded the homework, that sort of thing. He would also need high school to be aware as you say and transitioning him on school action with a student profile would be protective.
Processing speed at 4th percentile is significant. Low processing speed scores can be caused in two ways, ironically. It can be 'genuine' slow processing where a child needs time to consider and respond and is deliberate, accurate, but slow. Then there are also a number of children who get low processing speed scores because of attention issues - they are fast but slapdash, or cannot sustain concentration over the 2 minutes required to complete the measured tasks and 'drift off' half way through the task, or panic about the time limit and over focus on getting as much done as possible without being accurate. It would be really helpful to know which sort of low processing speed score your DS has in terms of how to support that. Again, information to high school that either he needs additional time to process spoken and /or written instructions, or consideration of his attention and focus difficulties would be crucial. If he has really slow processing (as opposed to attention issues) coupled with working memory difficulties then he certainly will need mindful teaching at secondary and should definitely be on the SEN register IMO. I would be fighting for that if I were you, rather than requesting a statement which would undoubtedly be turned down in the light of your sons' scores and high attainments and the fact that he hasn't had support at school yet. What do you want for him? If you want a significant amount of individual support in lessons then fighting for a statement might be worth the fight. If you want him to have some support in school particularly around organisation, mindful teaching, and the chance to be supported to develop some strategies to handle his difficulties, TBH I would think one to one would not be supportive of that (evidence is that high levels of TA support doesn't help independence unfortunately), but he DOES need people to know his needs and some degree of accommodation and teaching of strategies to address them. As he gets older the Sheila Cottrell books around study skills might be helpful.
I'm not sure what the EP meant about higher order processing deficit...higher order language skills are usually around inference and deduction, kids on the spectrum often struggle with those skills. But the 'like being deaf' thing makes me wonder if he is talking about auditory processing which usually has a knock-on effect in spelling skills, which you said your DS doesn't? This link has lots of useful info about auditory processing
www.asha.org/public/hearing/Understanding-Auditory-Processing-Disorders-in-Children/
Best of luck trying to get things in place before secondary school.