I'd argue mobility involves more variables than just the width/length of your stance or how far apart you're feet are - things like weight distribution for example play a role, otherwise I agree with you. Opinions vary when the topic comes to mobility, at least from a stand up perspective, as I'm sure everyone here is already aware of.
The best mobility is in my opinion ensuring you have a 50/50 weight distribution with your centre of gravity equidistant between both legs you want a straight (you want good posture to) at least from the karate perspective. Your stance shouldn't be more than a just over a shoulder width apart - in regards to mobility/leverage imho.
As for actually foot placement, generally you don't want to be too wide/too narrow, too deep or too tall lol when you consider mobility/balance/leverage - there's a optimum intersection between the above. People's opinions on how much 'is too wide considered,' for example vary.
So in my mind for best mobility - 50/50 weight on both legs, no more than just over a shoulder width apart in regards to foot placement (i.e. so you're not wide/narrow) - that's a general limit I'd say, as you don't want your feet to close together or too far apart.