Cisco hasn't publicly committed to what's in FMC 6.5 yet (due out in the Fall). I know they do want to increase capabilities of the FMCv.
It's not so much the sensors themselves that will gives your manager problems scaling - it's event ingestion. That many sensors can result in quite a few events per second if you log connection events. I'd suggest looking at either not logging all connection events or sending them to an alternate logging host.
You might also seriously consider Cisco Defense Orchestrator for management of your devices. It can now ingest events and manage FTD.