One thing to bear in mind is that there is a mysterious, unknown formula for how much wpf you need per point of Power Throw in order to offset your armor.
So apparently you need 13 wpf per point of power throw in order to take full advantage of that power throw. If you have 100 wpf, then 100 divided by 13 says you'll utilize a max of 7 power throw.
But that's assuming you're naked.
Armor reduces your effective wpf. The exact formula for this is unknown (every posted formula I've seen is wrong, by a pretty wide margin), but you'll get a message if your wpf, minus armor, is not high enough for your power throw. So your best bet is to decide how much power throw you want, then decide how much armor you want to wear, work your way up to that, then start adding points to throwing proficiency until you stop getting that message.
As an example:
Norse Warlord Helm
Banded Armor
Scale Gloves
Mail Boots
Power Throw 4
=
104 wpf
I went up to Power Throw 5 and was able to fully utilize that by downgrading my helmet to the Nordic Huscarl Helmet and my armor to the Mail Shirt. Otherwise I would have needed more wpf (how much, I don't know).
To answer your basic question though, as far as I know, throwing proficiency only impacts your accuracy. It's a big impact, though. It might also impact your throw rate but I'm not positive (i.e., how long it takes you to go from a cold stop to a throw).
