So far I'm using them in my mystworld mob farms/grinders. Didn't know about the pipes but my turtles sit at head height, with transposers in the floor beneath them, so I'm not sure I'd be able to get all the drops if I replace the transposers with pipes. Pipes might stop the XP flow too [in my design]. I had the turtles on the ground and they were stopping the XP orbs.
Also, my design isn't optimized for spawning rates. My cages have T5 solely because I can redstone them off, which allows multiple spawners per enclosure/spawn area. And my turtles can't keep up as it is. The only design I utilize that's optimized for spawning rates is a Blaze room [11x11x10 ID] where they fall into water and the rods get fed to my generators via a router.
When I parsed your code, I envisioned the turtles holding onto the drops for a little while before dropping inventory. When the grinder is running, this isn't an issue. But when it's shutoff, the turtles will hold their inventories until it's turned on again. Which could be an issue, depending on how badly you need those drops [worst case - each turtle retaining 15 stacks of the desired item]. In particular for the pigmen, I have 2 turtles that craft swords/nuggets into ingots and it's best that they don't get too much all at once or that system gets clogged.
And I seem to have an uncanny ability to necro-rez just about any topic.
Another thought that's beyond my capabilities for the moment, would be to keep your code as-is but have the turtles be wireless. And with a wireless computer nearby, have the computer issue a dump-all to the turtles when the grinder is shutoff. This way you'd max attack time and get all the drops too. I plan to work this idea into my farms as sometimes the turtles don't load properly and it's kind of a pain to check each one to see if it's running or not. My original idea was to have a computer at each farm rebooting the turtles, with possible confirmations from the turtles, prior to starting up the spawner. Once I get that working, shouldn't be too hard to throw in a dump-all too.