Yeah definitely light intensity is to much. Can you rotate your plants at all? Or zip tie your light directly to the ceiling somehow? like no hangers....Not sure what your feed is like, but heavy feeding this far along could also play a role here.
Awesome man, you'll love it. The biggest advantage the 320 has over the 300 is the dimmer knob. The 300 does not have any markings on it to tell you what percent you're running you're lights at where as the 320 does. The 320 also has a higher CRI; you'll be happy with either choice!
I purchased two GLS 300 lights last year, first led lights I've ever used and owned. I'm incredibly impressed by them, by far best light I've ever used. Gone through two harvest with them and they are killer! I believe you're going to be satisfied with that light.