With warmer weather around, I finally was able to get the car out this year. The more I drive it, the more convinced I am that my shifting issues are the transmission, and not the shifter.
Aside from the 4-5 shift being difficult due to the gate placement, it's also a bit finicky even once it's in gear. You really have to force it ALL the way into 5th, otherwise it will pop out of gear. Even once I do that, it will occasionally pop out of 5th under deceleration, even if it was comfortably engaged before then. I'll just be driving along, and all the sudden I get 4000 rpm and no acceleration. I end up having to drive with my hand on the shifter all the time, forcing it into gear.
I suspect the trans has had a hard life. I just bought a new F23 from the DF store, and will be undertaking that project soon. I already have a new long-block engine, might as well have a new trans too, right?