Big Jim was certainly a great guy and he did a lot for our hobby but he didn't know everything and he wasn't always 100% right. The Monster IS always faster in sedan. I don't race off road so I can speak for that class but on road the Monster rules.
Why does Trinity still sell the P2K2? Because it is a good motor and guys like Random still buy loads of them. There is no sense discontinuing a product there is still demand for even if it is not as fast as the new stuff.
As was said before the P2K2 is a great motor but if you look at the A-Mains of big open motor stock races you will see the entire A and B filled with Monster equipped cars. It doesn't take long for racers to figure out stuff. If the P2K2 was faster everyone would be using it.