I find Blade Spawners to be a solid, effective unit. However, I don't find them to be overpowered. Yes, they can clear a 50-60 ship planet that's been non-alerted very easily, and can do the same to a 150 ship planet eventually.
I'd expect the same out of a full cap of Missile Frigates I/II/III.
There are ships immune to blade spawns which can make you crazy, but they're few and far between and I like it that way.
However, I've tried the tactic you're describing, in a few different formats, against significant defenses (200+ ships MK III+, 500 ships MK I+). I've done kiting, cloaked transports, the works. Eventually they just can't kill everything coming for them and ships that are 'cutting the corner' will eventually catch them, they're not THAT fast. From a time-basis however, how LONG does it take them to kite the system for you? If it's 30 minutes to clear a system, I don't find that overpowered. I find it a waste of firepower. I could build a fortress or two for the same price and same general effect. Blade Spawners ain't cheap, and you've spent 8500 in Knowledge to get there.
I wouldn't mind if Blade Spawners were made more 'glass cannon' ish so that if anything was able to finally catch them they'd die in a puff of logic, but I'd hate to see them nerfed from a firepower perspective. With the new reinforcement changes lowering the volume of AI reinforcement spawns of them, my largest concern (them in the hands of the AI during early game when you've only got MK I to work with) has been mostly removed. Now, once their targetting gets cleaned up and they stop having issues against guardians/guardposts under glass and having an addiction to shooting at the forcefield itself, I might agree with you when I see that in action.