Yes, there can be a disadvantage to using a smaller subdiv cache. It might slow the rendering speed if you have a scene with lots of reflections, or rendered at high resolution and/or detail. It is more of a problem if you are rendering with more threads, because each thread is only allocated a small part of the total subdiv cache. In general it's difficult to predict what will be the best size to use. If you're working on a long-running project, it's probably worth spending some time testing different setups. Otherwise, stick to the default value and maybe reduce it if you get errors during rendering or if you know TG is using too much memory.
Matt