As long as it isn't tagged and released, you cannot expect it to be stable and finished with backwards incompatible changes. Your question isn't accurate, 'is the pmmp alpha9 stable enough ?', stable enough for what?
I would highly recommend using a tagged version, so ALPHA8 in this case. You don't know what changes are yet to come in ALPHA9.
more precise: don't even use 3.0.0-ALPHA*, they are all pre-releases. Use 2.1.0 to be more precise: don't even use anything in 2.0.0 or above. Use PocketMine 1.4; it was the last "stable" "released" verison. to be even more precise: don't even update Minecraft, the first version you play with is the best version you have ever seen.
You're soo right, at least if it comes to performance/optimisation, because apparently, that's one of the things Mojang/Mico$oft doesn't care about anymore...