I topped out at sub 10.3, personally. I was a 4 year varsity soccer player in HS. I fumbled around trying to play NCAA DIII soccer and then called it quits and stuck only with track. In HS I broke 10.8 fat/wind legal at a NEastern state. No one ever SNIFFED me on the soccer field. In fact, to this day, I don't understand why someone didn't slide in and say "with your speed and ability, I could make you a star", but I always seemed to slip through the cracks. One of my great regrets.
Certainly the comp. I faced wasn't necessarily high, but it also wasn't close. In know HS I DID face some legit comp. here and there, and I (at least individually) tended to wipe the floor with them, particularly from an athletic perspective. But my teams would often get crushed because I wasn't in a huge/strong program.
Soccer is weird in that athletically you can be head and shoulders above, but if you haven't found your niche or the team isn't built around exploring your niche, you'll be highly ineffectual. Unless your niche is SOCCER in general in addition to great speed, which changes the dynamic a lot more.
For example, if you put a bunch of 4.3 American football type guys on the "pitch", you might see some quick positive results, but if all they can do AT BEST, is look for some through balls with space allowing for separation, then the OVERALL opponent defense can smother that REALLY quick....unless the 4.3 guys have legit touch and intuition on soccer ITSELF. But, even an idiot with 4.3 speed has a punchers chance, so that can always keep things interesting, at least in terms of a one-off scoring opportunity. The game is more complicated than that overall, though, particularly the higher up you go. In my personal experience, I thought I could understand the game plenty and my speed was game breaking but I just never/seldom seemed to be in lockstep with others, so my own moments were fleeting. No one ever looked to help about it either....maybe a good example of why the US doesn't explode internationally in soccer.