Band Power

16mm should provide more power but will likely make it handle a little differently, e.g. harder to load. But 16mm doesn't sound outlandish for your set-up.

My 95cm roller speargun came factory equipped with a 6.75mm spear and 16mm rubber (actually measures 16.5mm). I wouldn't want to go any heavier than that

I bought a thicker replacement rubber only because it was on deep discount - I'm not looking forward to using it! I would rather try a 14 or 15mm rubber next.

Loading my 95cm roller is not like loading my regular 90cm speargun, it is more like loading a 120cm speargun, because you start stretching it from the muzzle. I've only just started using it, so this is still quite new to me.

BTW For non- roller spearguns, it is normal to use slightly longer bands for thicker diameter rubber (something than is often overlooked when comparing band power). I would think the same could apply to rubbers for roller spearguns but I guess you could fit whatever length bands you want.

With a roller, you usually have some discretion over how much pretension you set - Rob Allen has some recommendations for this in one of his YouTube video. He uses a heavy duty luggage style scale to measure it. I think he had recommendations for 14mm and 16mm bands - which I think are the only size options they offer as standard.