I have put a hydrofoil on every boat I've ever owned, and have seen all kinds of improvement. Especially with faster on plane times and lower speed handling. The boats I've used them on include: 1. Mid-70s, 16.5 ft Alumacraft with a 25 tiller motor. 2. A late 80s Smokercraft with a 75 horse Mariner, and a 2004 Lund Fisherman with a 115 Yamaha 4S.
The benefits are different between the boats. The small and light Alumacraft had a significantly faster top end with the hydrofoil, but a part of me wonders if that wasn't just compensating for not having power trim, regardless, it went about 7 miles an hour faster with a 2-4 person load in the boat and was about 3-4 mph faster with just one person in the boat. The Smokercraft had a terrible chine-walking problem, probably because it was the only of my boats at max HP. The hydrofoil tamed that quite a bit and didn't seem to really affect top end speed. The Lund's benefits are really noticed at lower speeds while on plane. The thing used to "swim" and was really an effort to control at ~10-14 mph. The hydrofoil had it on plane with less throttle and lower speeds which really helped control. It also seems to blowout less at WOT with similar trim levels when turning. However, I think I also lost 2-3 mph top end speed with it.
At one point in time (like maybe 15 years ago) I had pretty well documented the performance of all the above running the boats with and without the hydrofoil. Needless to say, if I buy a different boat tomorrow, I will buy a hydrofoil for it at the same time.