Agility based PT is a baaaad idea. I appreciate the thought, but you can create a 10 PT, 10 WM, 10 athletics pure thrower with that change. 190 wpf AND really high damage. They get close? Run for it.
I wouldn't mind the change, but it would make pure throwing too powerful.
The thing is that character would still not be that powerful.
It would have the massively reduced ammo limits (compared to other ranged), far from high damage compared to the likes of the top crossbows or 2handers, you would be unable to have much armour as the wpf would quickly drop below the 150 requirement for the 10 Power Throw, no wpf in another weapon for the same reason, you would have no powerstrike, and you would still have a very low range compared to other ranged.
Having agility 10 means nothing at all when you've got no strength, no power strike, no wpf in melee, no armour, and most importantly NO WEAPONS after a few hits on flesh or shields or simply having ammo going missing or being out of reach.
So yeah quite simply put I doubt a full AG dedicated thrower with 10 PT, 10 WM and 10 Ath would be overpowered. Yes it would probably kill someone who runs straight at it with no shield (much like most ranged do) but in doing so the thrower would use maybe a quarter of their ammo.