Miaodao is definitely the most OP 2h; it never seems any slower than the longsword, since it can hit without glancing even earlier due to the higher damage. You can't walk 10 ft without seeing a +3 Miaodao now.
Funny how 2h were always OP because of the lolstab, and you quote one of the rare 2h weapons without a stab as OP.
Never having to worry about an insta thrust from this weapon makes it easier to deal with. Most people underestimate the reach though, so they get hit for max dmg, and from such a high damage sword, it sure hurts.
Miaodao is definitely a good 2h weapon, but not the best.
You have to give away the 2h thrust wich gives you :
-cavalry disuasion (damn, he's got the thrust, too risky trying to bumpslash him)
-good ganking ability (when surrounded by teammates, the thrust is the easiest to land on one ennemy. Sideswings tend to hit allies, overhead can kill teammates)
-an extra attack pattern for duelling
I use a +3 Miadao myself, since quite a long time, on a 24/18 build, and of course it's great. But there's always thoses situations where you really feel the lack of a thrust.
Concerning the OP, i wouldn't mind a slight rework of the longsword, but... slight. As in very slight. I don't see it as that much overpowered.
Maybe the -1 speed, and +1 weight can be enough, without touching the damage though.