I urge extreme caution to the Devs concerning changes to P.I mechanics. If you make it too difficult or two costly to capture planets the game will be slowed down even more as conquest will become harder. Just as importantly, the AI's will have even more problems capturing planets than they do now.
I'll tell two invasion game stories from playing vs Godlike's OP AI's. Game 1 was a while ago, I think during some 1.6 beta. I had a great start and great luck with capsules and such and an unfortunate genius gave me P.I. tech instantly on turn 17! Well, I think lets eliminate the nearby Drengin menace and I send whatever ships I have (mostly from graveyards) over to him and also make 4 transports since I only see 4 planets (couldn't talk to him as had just traded). By T25 I took all 4 planets as they were mostly or very lightly defended. But hmm he had another planet further away.
Soon the Drengin bought a Merc and it and some weaker warships destroyed and chased off my meager fleet and now by about T33 he took his planets back with transports. Stunning play from an AI and just what I'd expect playing Godlike due to their huge economic advantages. About 10 turns later I came back with better fleets with Harpoon missiles and finished off his fleet and then my next batch of transports eliminated him by about 1 year into the game.
But this current Godlike game leaves me wondering what's up with AI's and transports? This time I was latish to P.I. T30 but had a decent fleet and went after the Yor with my fast transports arriving in their home system ahead of my fleet. Their planets were undefended so it was trivial to grab a couple planets but then I noticed that they had 2 of their own transports but rather than land them to retake their planet(s) they went off in another direction but then returned to be destroyed several turns later!
The AI's need to be taught to better use transports. Building ships with two transport modules and filling them up to at least close to 6 pop would be a good start, and just as important would be putting a couple decent drives on them so they have some speed.