It depends on what you get. The SE-Sport SE300 or whatever it's called mostly just helps on planing. The wider, more wing-like ones help both ways.
I have a V-8 in my I/O so it's not like a can't get on plane but the planing time is cut in half with a stabilizer thingy on there.
The reason I mention the SE300 is that I got one and it didn't help the wake like I had seen the Doel-Fin do on another boat. So I made my own with the holes of the SE300.
I noticed that you don't need a real hydrofoil to help with planing because the outdrive is angled downward with the trim all the way down. So my new stabilizer plate thingy is just flat.
Here's a link to a picture: http://www.bayareamotorsport.com/~jon/pics/boat/plate1.jpg