what effect does reducing this value to say, 5, have one the accuracy of the values recorded?
The multi-sampling is usually used as a way to filter out noise. Whether it will have a big impact on your system depends on how much noise is present (which is dependent on the hardware design and environment), and whether the higher-level algorithms (and people) in your code also do anything to handle the noise. For example, I've seen rocket motor thrust measurement setups that apparently plot the raw data, and let human perceptions do the noise filtering (
I'd work backwards - try adjusting your sampling to whatever sample rate you actually need (or even several times that), and see what it looks like. (it may depend on communications speed and memory availability. Best case is to record all of the 80 samples/s if you can - you can ALWAYS do the averaging in post processing, right? (I guess you haven't said whether you have a post-processing phase, or are doing things in real time...)
How can I tell whether 10 or 80 samples/s are selected?
Ugh. Awful documentation on that module. The rate is controlled by the RATE pin on the IC (pin 15) - GND for 10s/s, VCC for 80. From the DFRobot schematic, it LOOKS like they have 10k resistors going to both VCC and GND, and probably only one of them should be present; but the photo of the module shows them both present (I think; they're not labeled!) Grr. It's all self timed; if you're averaging 25 readings in 0.25s, it MUST be doing 80s/s, right?