I had 18m, but sold it after I rode it side by side with my Sonic 11m. I did not feel I was getting any benefit from 18m power and it was a bit harder to handle. A15 is not the best depowering kite, so the range will not be great, perhaps up to 12 knots to be comfy. I'd recommend racing bar setup with lots of trim available. It is slower (across wind window) kite than racing foils (obviously) which also means that apparent wind generation is not as efficient. Relaunch is good and it is easier to save from dropping it due to lower span compared to racing foil.
Locally, a friend uses 15m A15 when I use Sonic 11m. We seem to have the same low end on similar HFs. He has better chance to relaunch.
Since you already have Cloud 17m, your gain will not be great. If you do decide to get A15, spend extra $200 and get ultralight version, it should give you more advantage. That's what I would do. It is tough call between the sizes, but as an anecdote: mentioned friend had 18m and exchanged it for a second 15m (now he has two) with argument that 18m was just too slow to be useful. (However he weighs a bit less than 85kg; to muddy the water even more )
Wish i could be more helpful.
PS: If you go for A15, be ready for some surprises. Mainly almost non-existent bar pressure and reluctant turns - I usually loop it into the water if I have not flown it for a while. Long bar (60cm) will help a lot. Clouds turn well so this might be an eye opener.