Hi Jonny,
In the scenario that you have illustrated, you are nearly correct. Your ratios DO remain the same, however if you consider the radius of the countershaft sprocket as a “lever”, the shorter the lever, in this case, the more torque or "force" you will apply to the chain.
Back in the 70’s when I was doing a lot of Karting (Sprint and Enduro) we found the kart pulled harder out of the turns when we used the smallest countershaft sprocket that would allow the chain to “live” for the duration of the last practice session (to get rid of the chain stretch) and 1 hour race. The top speed was the same with either sprocket as verified from the maximum RPMs. Sprint proved to be the same but wasn’t quite as noticeable because of the abundance of torque due to the lower gearing. Again, maximum RPM proved the same top end speed.
Since the very small benefit in torque you may obtain from the smaller countershaft sprocket won’t make any difference in your top speed, you should use the largest sprocket combinations to keep your chain alive.
Hope this helps a little.
Yours in Sport
Terry A. Hume