I've shot a lot of footage at 1/100. For really slow-moving scenes (dialog-heavy), honestly, nobody can notice, except for maybe DPs. For action-scenes, the difference is rather pronounced. I think even non-filmmakers can tell that something is just...off.
There was a ways back when we had a BIG-ol' debate on the merits of the 180-rule. I really just wanted someone to explain to me WHY we followed it -- what was the link between shutter speed and frame rate?
The general consensus we reached is this -- there is no link between shutter speed and frame rate (except that your shutter speed obviously can't be slower than your frame rate). The 180 rule exists simply because of the history of how cameras were made, and more importantly -- audiences have grown accustomed to it.
As a general rule, it's good to keep your shutter speed at or near 1/50 or 1/60, simply because it creates a motion blur that audiences accept as normal, and anything different will weird them out, consciously or not.
Disclaimer: there's always an exception to the rule, Private Ryan, yadda yadda.
Anyway, to answer the OP's question -- if you can't up your f-stop, yeah, you're gonna need more ND filters. Keep in mind that this of course is gonna keep you at a really shallow depth of field; maybe you want that, maybe you don't.