When you add a Futaba FASST module to a 9C transmitter, you will find no degradation in latency. However, it won't speed up, either, because latency is determined by the processor. That being said, though, because you need to set the transmitter to PPM mode to use the module, you'll have a somewhat faster latency because PPM mode is a bit faster than the PCM mode.

If you're not flying complex CCPM helis, then latency becomes rather moot, since that's where most testers have concerns.