The tp bonus is at 100% TP your WS behaves like it has 200% tp, at 200% like it has 300% tp. 300% tp is the same.
This really wont help pentathrust at all since penta thrusts tp modifier is accuracy. This really helps Wheeling Thrust and Impulse Drive where the modifiers are defense ignored and damage multiplier respectively.
Actually Skewer's TP modifier is accuracy.. Pentathrust's only modifier is doing more dmg as you have more TP, but this will still probably be more useful with Wheeling Thrust and Impulse Drive.
Is it me or it should read Weapon Skill: Polearm? Please correct this. By the way if anyone have more info on what is the TP bonus alike, make a post or send a reply to this post.
As for the TP bonus I am not sure about, but from what I have ready on the Martial Sword page it looks like it either adds 100% tp or 1/2 of your current tp to your current amount when you use a weapon skill. So it would be like this at 100% tp you would have either 150% or 200%. The only thing not known is if it capa at 300% or not. If you want to look at the posts yourself go to the martial sword page it is all there