Short answer: in 15-18 knots, medium wind, a Hyperlink 7 is fantastic. I can use it to 20 knots.
18-25, you'll want a 5m foil kite. 25knots will be too much and you'll be on a micro foil kite like a 3,5 concept air wave.
Long answer :
I was using a quiver of North Monos my first 2 years in a hydrofoil. 7-9-12.
Then I got a Hyperlink UL V1 9m. It was much better for foiling, no fluttering, better range, better depower. Stopped using the 12-9 Mono. A few sessions I was using the 7mono and it felt so jerky and flapping after getting used to the hyperlink. It was terrible experience. ( I was still using Monos on surfboard, but by 2nd year foiling, I stopped using surfboard as I was ridden shitty waves we have on HF.) So I got a 7m Hyperlink. I use it 16- to up to a bit over 20 knots. I find them much easier to control and depower than the Mono.
Finally I bought a used Sonic 2 13 for 8-12 knot sessions ( Hyperlink 9m works well at 10 knots) but too much trouble below 10.) The Sonic was really amazing but too technical to fly and the bridles so long and complicated. So this summer bought a new Hyperlink V2 13m. It goes well in 8-14 knots.
I don’t agree that (Hyperlink) foil kite is difficult to control power at top end, on the contrary they are easier to control power. Also, I can switch from 20m to 15m lines and get additional 2 knots top end, but I rarely do that.
My conditions in 16+ knots is choppy, short period windwaves and confused water, so not easy.
Recently I bought a Concept Air Wave 4,5m for 20+ knots. I've not used it yet. Soon though as fall winds coming.
I've become addicted to the smooth power and ease of setting up and not pumping. I kite solo often, so it's easier to launch and land solo.
That said, I'm the only one around here using all foil kites, everyone else is on lei in 12+ knots. The new Slingshot SST, 2019, looks very good, it seems to flap less than my Monos did.