1. It makes no differance if the plumber used plastic or C/I pipe.
2. It makes no differance if the drain line is 3″ or 4″ for each toilet.
3. If you’re area has plumbing codes and the plumbing has been inspected, I’m confident the work is acceptable.
4. As for the toilets, this 1.6 gal. per flush regulation was thrown at the plumbing industry without time for proper testing and experimentation by manufacturers. Initially the toilets were poor flushers, but each year (as a whole) they have improved. If I were you I would replace one of your toilets with a new one, if you find it to be a substancial inprovement, replace the others. As for the best toilet, if you check around YOUR AREA, people will have suggestions.