Thanks again everyone. Guess I will just have to actually use it and see.... but that requires the weather to co-operate.
Setup with Baader LRGB-H-a (7nM), with Astronomik CLS LP filter in front of it all. Might have to use an UV/IR cut as well in the train, but trial will tell (have written SBIG to ask if they used UV/IR cut in their front window back when this was made)
I looked back over some old subs I took with the 9um ST-8Xe, ina VERY similar setup, and those stars were borderline blocky, so 7.4 can only improve upon that.