I've never really noticed any effect from the running line. I definitely think that all running lines are not created equal, but I've never experienced substantial differences in my casting from just the running line. I would say it's a casting issue. I believe you are a scandi or skagit guy. The difference in head length will take a bit of getting used to, but I don't think the learning curve is all that steep. One consideration might be line weight. When you look at Airflo line recs, you will sometimes see differences between Dec's recs and Tim's recs. I have tended to go with the upper end of the line recs because a heavier line seems to match my casting style. You could easily try out the 7/8 and see if there is a noticeable difference. Of course, if the grain weight of the 7/8 is well above the grain window for your rod, that would probably not be a good idea.