I am familiar with the rod and line, the line will definitely load the rod but only if you can aerialize a long length of line. This is true of the steelhead mastery taper even when rod is one rating lower, but of course less line needs to be in the air. I would guess that the rplx 9wt will require about 40-50' to start casting itself. I use the 8wt steelhead taper on an Sage RPL 8.5' 8wt. It's a pretty good match.
The line handles very well, mending and rolling, even singlehanded Spey casting, etc.
Coming from a shooting taper it's a big change. Another option would be to use a shooting style floating line, I've seen some out there. This line wouldn't handle well though, unless short lining (small streams) or straight lining (coastal or stillwater).
Are you going topwater for coho this fall?