I'm pretty sure that without FS ships 900AIP is the point where practically any defensive line crumbles. I've got every turret mk1-4 bar lightning and flak, 4 sets of Mk V turrets, 1k fleetships on low caps, five modular fortresses, eight champions, HRFs, roaming enclaves, radar jammer mk2 and (briefly) a botnet golem.
AI can roll through it. Last waves were ~26k ships or so half armour half anti armour. This wave was just 33k anti armour. three mk3 lightning warheads didn't even make a dent. :L
Still. It's fun seeing how many ships you can get a defense line to hold against. It's a shame most of the golems are on the wrong side of the map, and that I did a stupid and forgot to do the FS campaign which pretty much doomed the enterprise from the start.
Thinking about it. If you could get a gravity drill on that planet somewhere far from the AI's entry point...
That or roll a "string" of planets that you can use as a 2-3 planet thick whipping boy with full caps of turrets on each.
Edit:
http://cloud-4.steampowered.com/ugc/585783733674279341/9DC0D6DD75BACD5BB846BFE7987B9671230972E0/One last stand.
Edit 2: The pricks popped the nuke before it jumped out of the system >.>