The first set of vader shins i had were made from grp, they are a little heavier but certainly not flexible.. They cut into my feet when walking up stairs. The lightweight abs pair i have are not only more accurate but way better to wear.
As for trooper armour, there is a guy here in the uk selling grp armour parts, looks like fx style. I would personally stick to wearing the abs tk stuff, i think that grp is going to pinch really bad and i think sitting down will be a big no-no come to think of it i think anything apart from a static display would be a no-no.....
Maybe someone here has tried the grp trooper armour and can enlighten us even more