If you check the linear progression, +4 or +5 speed makes perfect since. As you go along, the speed rating usually drops 3 per bow. But from Rus to Longbow, it's a drop of 9. If you take into account that the Rus only drops 1 instead of 3, then the Longbow by pattern would theoretically end up at 54 speed rating. Since all its other stats generally follow a linear progression, this off handicap makes it underpowered. Though if you wanted to buff damage instead of speed, that could also make up for it. But I'm betting anti-range would whine a lot harder following a damage buff than a speed buff.