I recently purchased a Farlows Holdfast F 11"3' 7/8wt "switch" rod. I contacted OPST for a line recommendation. They recommended a 325 grain head because the rod is fibreglass.
I'm not an expert caster but I've been using a 275 grain OPST on a 10' 8wt Hardy Demon with very good results. My results with the 325 grain OPST on the Farlows have been disappointing to say the least. I don't know if it's my unfamiliarity with fibreglass or if the head is too light. To me the head seems much too light. Not being in a position to buy a pocketful of $55 heads, I posted a message here asking for a recommendation but didn't get any replies.
I then discovered your videos of your Farlows 7/8wt spey rod, in which you used a 430 grain head. So I'm writing to see if you might be able to offer a recommendation for my rod.