I've been testing my Flex 6600M with the latest versions of VarAC and VARA and I haven't found any latency issues. The modem seems to work fine with a range of filtering including narrow filters which cause an increse in latency. The latest version of SmarSDR was used along with DAX during testing -- the OS is Windows 10/64.
I measured the modem's shortest TX/RX switching delay at roughly 100ms and that seems to work fine with the Flex. I suspect those who are still having issues with Flex transceives might be experiencing OS system latency -- maybe someone can shed some light on that possibility.
I recoreded a short video that shows the Flex in operation with VarAC v3.0.2 and VARA v4.5.5.