is there a big difference in performance of cat5 2pr and 4pr?

Answer 1

For 100 mbps and slower systems only 2 pairs are used 1-2 & 3-6. For copper gigabit, all 4 pairs are used. Gigabit is 10x faster than 100 mbps so yes, it would make a difference if all of your equipment was gigabit.

In 100 mbps and slower systems even though only 2 pairs are used, all 4 pairs are still connected. They cannot be used for any other purpose if that's what you're alluding do.