worth upgrading from REL Storm to REL Storm III?

I would say probably yes, I had a Storm and Storm III at the same time. I've since sold the Storm (didn't need an extra sub). The Storm is a great sub for the money they go for used, but the Storm III is somewhat better. If you're using it for home theater and music, the Storm III permits dual connection (hi and low) so you can have LFE separate from music. I think integration is also a little more flexible on the III.

My Storm MkI also have dual connection (hi and low)actually 3 including balance input.

true, but as far as I know, you can't use both high and low inputs at the same time. The crossover on the mk III allows you to drive the LFE using low level inputs off the sub connection from your pre-pro, and the music LF using hi level connection to your power amp, and do this simultaneously.

I may be mistaken here though.