I'll chime in on this one a little. My son and a kid that he plays HS and summer ball with both throw roughly the same from the mound. My son is probably a couple MPH higher. Son throws mostly 85-87, on good days 86-88 and has touched 89 a couple of times. The other kid is 85-86 hitting 87 at times. I have seen them long toss before. Other kid can throw 300' pretty regular. When they go out that far, my son usually winds up 1 hopping it. Other kid throws more over the top, my son is more 3/4.
Personally, I kind of cringe when son attempts the 300' throws. I see his mechanics change and I worry about his elbow. I also know his mechanics are different when throwing in the field than from the mound. He plays SS and drops down a little more from the field than from the mound. The other kid has a cannon from the outfield, but my son has not played OF since before HS and even then it was limited. He has always thrown harder from the mound than from field as well.
Can my son throw 300'? I'm sure he can. Do I think it is a good thing for him? I'm not a certified pitching coach or anything, but I don't think it is good for him. Others may feel the same, I don't know. Maybe that's why some ML teams don't like their pitchers throwing that far. Just some of my thoughts.