Setting a 300Mbps limit works as expected.
I would just go back to the original firmware but the only reason I tried the op24 version was because I was having a problem with some cameras lagging and disconnecting and that hasn't happened since swapping to this firmware. It may not have been enough time to tell though.
EDIT: Disabling software offloading makes things even worse bringing the maximum SQM speeds down to a little over 400Mbps. Enabling software offloading allows it to return to the 600Mbps limit. All of this with a download set to 1000Mbps.