I think it might be interesting if they did not link PQ with pop cap for two reasons off the top of my head.

1) Lets say you have 2 planets both are 10QP and one is a desert planet and one is a temperate planet. The desert planet has 10 usuable tile because of all the flast space BUT no one want to live there because it a desert. So for that world have the cap 10 onstead of 16. What taxpayer wants to pay tax to live in a desert. The temperate planet has 10 usuable tiles but would have a pop cap of 16. Hey, it’s a nice place to live.

2) It would make ‘reading’ what other races have in pop harder to do. If I see a race that has 2 planets of 4PQ I KNOW they have 16 (8 and 8) total pop (not including farms). Lets say I have 3 planets of 4PQ, I then know I can out ‘attrition’ them if you will because I will have 8 billion more pop to work with. It would be much harder to get a read on an opponent if there was a range for QP          populations