If increasing the line weight to a six worked with the 5 wt for you, then you might want try a 10 wt on the 8wt as well.
I bought 3 rods in the last year or so, a 5 7 and 8 wt for an average manufacturers line rating of 6.66. But the average CCS rating for those same wts and models was 8.91. More than 2 line weights each heavier than the mfg.'s ratings.
But that is not surprising, since my main criteria for buyng a rod is light wt, hight estimated rod number and a large action angle for lighter lines. With lighter lines I usually throw lighter flies and like to be able to "tip cast" short distances and carry a lot of line for long distances. That combination works the best for me.
For heavier lines, 9wt and up I throw mostly big weighted flies, and like a lower action angle with not quite so high an estimated rod number for the line I am using. This means they don't have so much "tip action" and bend deeper into the rod with the same percentage load than what the faster- actioned, higher ERN rods do.
The truth of the matter is that to do anything in fly fishing requires experience and trial and error, and nobody can tell you what line fits what rod for you. And the line that fits a certain rod for you today may well be two line sizes two big in 6 years. Or a taper that feels perfect today may not in 2 years or even in two months.
The best thing to do is if you are in an area with a fly fishing club or a casting club, is to join so you get lots of casting time with others, and can try lots of equipment and combinations in a short period of time without having to buy it.
Or you can do how most of us have done, and just fish - enjoying it most of the time and swearing the rest.