Its probably a safe assumption that later versions add functionality are to remain competitive in the industry (firmware changes tend to aim at fixing operational issues).
Whether that functionality is aimed at performance or other aspects is obviously down to each model and each manufacturer.
Personally I have come across specific models where a newer version turned out to be a retro-grade step as corners were cut for lower production costs.
It has always been useful to find a knowledgeable vendor who is happy to discuss these matters and make recommendations. Inevitably they are not the cheapest but I am happy to give them my custom following such service. And they in turn can see that I am an existing customer and thereby remain willing to answer my queries ~ a virtuous circle? :-)