My apologies for sounding wishy-washy, but it depends.
If you a highly muscular your BMI is going to be quite high. Athletes in general (especially football players and wrestlers) tend to have very high BMI's. But if you are not competing at a professional level, you probably want to get that a lot lower. It's a general guideline and useful for most people, but not everyone.
That said, 300 is probably far too much for 5'9". You might want to reasses the usefullness of BMI applying to you after you drop down to 230 or so.