For the set of starships "Zinth" and "Spire":
Things that inspire more "must kill that now": Ion Cannons
Things that inspire more "I'm going to loose a lot of ships": A few dozen Tech III/IV ships under a Mk 3 forcefield.
Things that inspire more "Must change tactics now!": A wave of ships from a Special Forces base, or re-pop, heading towards a pack of vulnerable ships (science vessels, or a newly constructed command center).
A Zinth starship just popped out of nowhere on an almost cleared planet (came through from a wormhole as a wave? Really no idea I didn't see it until it hit my command center). What did I do? Lassoed a third of my nearby force and said "kill it". It went down pretty quickly. That really should not be the reaction (or the effect) of something described as "a mobile fortress".
Yes, I was hitting it with mostly Tech II/III and a couple of IV ships. Yes, it had no extra ships around it for some reason. Really though the reaction was, "I'm not going to save my command center, that sucks", not "panic!".
I killed at least one Spire starship earlier in the map just after I pushed through the wormhole. The only reason I noticed was that it was on a tiny slither of health left after I pushed my ships through the wormhole waited a little for them to clear things out a bit, then pushed them back to my planet to heal up. Maybe 10 seconds and there was a huge pack of ships around it.
Really, the reaction to seeing the AI fielding a couple of those starships should be to react like it's an Ion Cannon in the early game; take them down as fast as possible since you can't afford the loss of ships. And even then expect it to hurt. The AI should react in a similar way, if they see the starships appear of this calibre, they should probably hit them with everything they've got. Since otherwise they know you'll just retreat and repair, something the AI doesn't seem to do.
As it is, the Raid and Flagship starships feel like they're worth it. They're both pretty chunky so they'll survive really well. The Flagship is an excellent upgrade over the Light starship. The raid starship does feel like it needs a bit more something. Some sort of bonus damage to forcefields (or being able to shoot things underneath them), or some sort of bonus to being by itself. It ends up just being tossed in with a pack of regular ships otherwise, and it doesn't have a "use this for raids" feel to it. In fact given it's strong vs bombers, it currently looks like it's much more of a defense ship then an attacking/guerrilla warfare one.
The Leech starship just doesn't feel like it's worth the cost. It's essentially a slightly slower Raid starship, but it has the parasite ability, but it takes 3 times as long to build. Parasite is really nice when there's lots of those ships in your force, but when the parasites start to get too low a percentage, you just don't get them taking over many enemies. I haven't played with Leech enough to be confident, but it doesn't feel like it's worth the cost of 8000 knowledge for a small amount of essentially extra resources at the stage of the game you'd get them.
In fact even when I don't play parasites as starting ships, I don't get the leech starships, so there's obviously something wrong with them!
I'm inclined to think that Raid:Leech starships should mirror the Fighter:Parasite pattern. Leech should have less health then Raid (since raid is more hit-and-run-and-repair whereas Leech should be in a pack of ships), a bit more shielding and take only 5-10 minutes longer to build then the Raid, rather then 3 times as long.
Also the Leech should either be cheaper on the knowledge (since it's a lot more expensive to build), or not depend on the raid ship to build it (or honestly they both should be seperate and cost 6000 after the changes suggested above, since that way you'd have something of a decision to make
). The Flagship/Zinth/Spire upgrade path works conceptually, but the Raid/Leech one just seems odd.