What 4seventy said!
How many shooters do you know who can consistently place 4 shots into 2 inches at 50 yds with open express sights, standing offhand? ...or 4 inches at 100 yds? NONE!

Obviously the rifle should be as accurate as possible so as not to exacerbate shooter error, but those quoted accuracy standards with an open-sighted double are unrealistic in practice. Off the bench, or even standing with a post support, or over sticks, maybe...
On required accuracy, 500grains is on the money IMHO, but it is nice know the rifle is capable of much better for that occasional long shot.

Regarding filing the sights, check your ballistic software. If sighted dead-on at around 90 - 95 yds, most of the NEs will not deviate more than one inch from the line of sight out past 110 yds or so. That's the 100-yd leaf on most heavy Brit doubles. All you need.