The heat thing is that you want to soften and shape the plastic, not melt it to the point of it sticking to the soldering iron.
You might want to consider going to a hobby shop and buying a wood burning iron... lower temps to start with and many alternate tips to play with.
$500 plastic grips?!? Where you buyin' your grips man?
I know what you mean... just yankin' yer' chain.
Have fun experimenting,