Somewhat strangely i think PPS scores less as it reaches the higher levels, I'm not sure why and i can't prove it but that's my gut feeling.
I use PPS primarily for it's knockback effect, usually with blover.
I think you should concentrate on getting bowling bulb to L7 as it's a lot better at that level and probably be a lot more useful than L7 potato mine.
In my experience bowling bulb seeds are fairly rare,
My bowling bulbs are about halfway to L10 and halfway to L9 and bowling bulb is the only decent plant in BWB world.