Is there a cap at hp so that eventually you will lose enough health to gain the most optimal firerrate and reload speed? Or does it still scale, even when you are down to 1 hp?
Yes, as far as I can tell, it does continue to scale firerate down to 1 HP.
I didn't find any specific stats, although by eyeballing it from this video (see around 1:25), it appears to be twice as fast at 1 HP than it is at full HP.
(Keep in mind this isn't hard evidence, only an educated guess.)