Would it be reasonable/ appropriate/ relevant to ask if anyone offered 10% additional time (where more was recommended) has been successful in this year or previous years?
I'm not sure how wide a range of dyslexia the 10% is applied to, and therefore how helpful to your case the answer might be. If I were in your shoes I think I'd put in a Freedom of Information request for some statistics for the past 5 years - that should give you time to consider whether there's any information that would be to your advantage.
If this is an own-admission authority school, I suspect they may be doing things 'on the cheap' instead of buying in a proper service. A Local Authority would probably have a small panel, including an EP, to consider individual requests for special consideration.
If it really is the case that they are applying 10% to a wide range of dyslexia, it seems a very blunt instrument, and arguable that they are not fulfilling their duty under the Equality Act to consider individual cases properly.
It is not, of course, a requirement under the Equality Act that a child must have a statement - although a statement, if it was in place, would form part of the evidence to be taken into account.