If your shutter speeds aren't a problem in practice, it isn't a problem. Using a shutter speed tester will only make you paranoid - the truth is that it's very unlikely that any shutter you own is dead on accurate at all of it's speeds. As long as it isn't enough to be noticeable in practice, there's nothing to fix.
As for the FM2/FM2n, I can only really comment that I've used a Nikon FM2n for the last 4 years and never encountered a problem with it's high speeds.