I think there are very few people out there who will call a ball out that they saw as in. If I can see court color between the ball and the line, it's out. I agree that the faster the ball is moving the more difficult it is to see. Also, perspective can screw it up. Bottom line, we're all just there to play and have fun.
Last time I disagreed with my partner on a close serve, we ended up giving the server two serves. No hard feelings at the end of that match.
I have come to the conclusion that I simply don't know what I don't know.