How does bluetooth sniff mode work?
To know all the details you might want to read the this document.
Beware that that documentation is extremely technical as it is intended for communications and system engineers.

But in a nutshell:


No sniffmode will not influence the datarate as it is not used while transferrig data to the device.
It is used in one of the idle/sleep modes when no active communications is ongoing.
Snifmode is used to see what other devices are active within the area.