Chipping and cracking are your enemy here. I file the tips down to a square on most of them, just a simple jigsaw cut, then sand. Never really taken more than that off though, i'd imagine a saw would result in some cracking the farther in you go if you aren't careful. Sanding and sealing is your best choice if you are gonna go for it though, dont recommend large cuts
But if you DO attempt it. Maybe you could try to wrap some masking tape around the blade right behind where you are going to cut it. That might minimize the chipping some They suggest you do that when cutting wood that you don't want chipping. It might not work cutting composite but, it couldn't hurt?
Odds are that there's a foam core there. I would recommend not trying it. If it was a two piece I would say go ahead and try it if you're willing to part with the blade if it doesn't work, but a OPS? Not a real great idea.