Plexi-glass is actually 7 times clearer than glass. That is why when you look into a plexi tank that it almost looks 3D like the fish are going to come right out to you. As far as cutting out the light spectrum I don't believe it will but someone might have more knowledge on that then I do...
I have been keeping reef tanks since the early 70's and they were around long before I started. In the 70's we could still buy good quality coral from the Gulf of Mexico that was covered with Christmas tree worms and all the stuff the aquacultured rock has on it now and a couple places in Tampa...