Don't forget that in modern era...adding 500 ft fences may not increase ISTP HRs..
Outfielders would play deeper, thus increasing xtra base hits, but not HRs. And if you think a Deion Sanders would have the speed to get ISTP HRs on more occasions, don't forget that a Juan Pierre or Raja Davis would be playing in the outfield and run down the line drives just as speedy as Deion runs the bases.
