On another note, I've been playing a 3.17 off-line game through to the end and I think I can say that the Sirian Doctrine is, indeed, officially dead. Not being able to use siege weapons from aboard ship made amphibious invasions extraordinarily difficult, even with Marines; you can lower the city's defenses with either Frigates and later ships or with spies, but your unit attacking amphibiously will still get very bad odds. Yes, you can still give Tanks Barrage promotions, but those expensive units will die in hordes if you use them this way; remember that unlike siege weapons, Tanks don't have withdrawal capabilities, unless you also give them Flanking promotions.
I myself abandoned attacking from aboard ship. I returned to landing my units before attacking, Marines included--they were needed ashore to protect the more vulnerable City Raider Infantry and siege weapons. So much for using Marines amphibiously, unless the city's defenders are so obsolete that Marines are overkill.
You really need Flight and Radio for Bombers to soften up the defenders, and of course, if it's an inter-continental invasion, it may not be possible to bring Bombers to bear, at least not for that crucial initial attack.
Frankly, this change almost feels like too much of a nerf to me, because it seems to have indirectly nerfed Marines as well. Or maybe I'm just whining because it seems specifically targeted to eliminate one of my favourite late-game military tactics. The thing is, the Sirian Doctrine wasn't just a winning tactic, it was a game saving one; it didn't just help me win, it also accelerated the win. We all know that the late game can become boring, and fast-moving amphibious invasions often helped push the game faster toward a conclusion. Now, domination and conquest wins won't just be tougher--which I think was the designers' intent--they'll be more tedious, too. And we needed more of that in the late game like a hole in the head.
I think this change should have been balanced out by giving ships some sort of collateral damage attack--not as much as Cannon or Artillery, but something.