Haven't done it but it appears as though this is a test that will guide identifying particular learning and developmental disorders like stuff to do with visual-spatial learning and auditory processing etc.
Sucks that it's expensive but tbh it's not the worst thing.
Honestly with late-diagnosed ADHDers and autistic folks it's my unprofessional opinion that you're very likely to see a... let's call it a lopsided skill tree. However you want to slice it, there's very often a good degree of intelligence or aptitude, especially in particular domains, and this is typically used to compensate for developmental disorders or deficits that exist elsewhere. Of course, some people are just straight-up ADHD or what have you and they don't have any other comorbid developmental disorders too but they're pretty common.
It's hard to say whether or not it's going to be of any particular use to you personally but it may help rule certain things in or out and it doesn't hurt to be thorough.
I also suspect that if the psych is going to personally conduct this test with you that they're probably doing that magician's sleight of hand thing where they're directing your attention on one thing in order to get you to ignore the real stuff that's going on elsewhere. I probably shouldn't spoil the intention of the test but, fuck it, I'd say that unmedicated ADHDers are generally going to be expected to perform well at the start of the test and when things change up significantly (e.g. going from visual tests to verbal instructions) but that there's going to be a fairly apparent dip in performance when the subject has either hit the wall (you know the one) or they are unable to compensate in a particular domain that is being tested where there's a weak spot. If the psych is good they're probably going to throw a few curveballs at you like interrupting the task of the test to get you to read an analogue clock or asking you about something that you mentioned in conversation earlier or asking you other questions which will pull your attention away, such as asking you to explain how you performed under exam conditions in school. This will be to assess your ability to refocus on the test and your ability to not get frazzled or to collapse in a barely-functional heap because you can't shift your attention in an even way. This will also be used to assess frustration tolerance etc.
Obviously this is just pure speculation but yeah, that's what I'd be expecting going into this.