I've found stealth battleships highly effective in taking out guard posts. They're effective at mark one if you're willing to flee early, but pretty ridiculous at marks two and three.
The key is to take out the tachyon guardians first, then send in the stealth battleships in low power mode. While they can't stand up to a moderate fleet for very long at all, they can slaughter several common types of guard posts before the AI has time to move anything else to within range. They can also handle some of the guardians fairly effectively as well. The key is not to hang around too long, and put them back into low power mode once the hit is done, so they'll go back into cloak and not waste that cloak on whatever tiny ships 1trickle in. And beware of enemy decloakers, of course. I've seen an unfortunate amount of those in my last game. If any are in yellow damage, it is pretty safe to send them stealthed back to your system and let the rest continue wreaking havoc. The trick is just in making sure you don't stay uncloaked for too long. Hell, if you're careful and lucky (setting priority targeting properly) they can even take out the tachyon guardians for you.
They honestly do have the speed and firepower you want, just nowhere near the staying power you'd expect from their health and 'battleship' title. Just focus on taking out guardians and posts, then hiding before the AI catches up to you, and you'll be pleased.
I haven't ever been happy with teleport ships on the -attack- but I've been pleased with them on defense.
