500fps isn't a solution. The logic isn't even sound. You assume the people who are cheating at 350fps are more resistant to pain, but will be somehow crippled at 500fps. It's just punishing everyone who calls it at 350fps.
And cheating isn't an excuse for being a dick. If they're blatantly not calling hits, then sure they deserve a few good face shots, but I'm not going to sink to that level, it's unsportsmanlike.
Some rulesets have this written, and for others it's an unwritten rule, that players will not intentionally aim for people's faces. It's common courtesy. Regardless of whether or not that person is being an ass, it's no excuse for you to be breaking rules either.
Now that being said, there are other, more sportsmanlike alternatives. The inner thighs usually hurt more and longer than face shots.
The one thing I've found has NEVER solved the problem, or ever helped the problem, is calling someone's hits for them. I've never seen a situation in which a player yelled at someone to call their hits, and that person called themselves out.
But it's completely different if you do it politely: "Hey, such and such, you've been hit man". It's happened to me, and I've nicely asked others to call it, and guess what? Works every time. No hard feelings, no resentment.
If we're talking about deterrents, having visible embedded admins at claybank worked great. If we got calls around an area, one of us would go there and all of a sudden no more complaints!
If we're talking about catching cheaters, then invisible embedded admins work great. Just have the admin flag on your back so your team can see you, but not the other.