I ended uo getting a 4K TV in the end (needed a new one anyway), so it has become a moot point, I just set Zwift to the highest resolution… 
In case others are confused as I was - the “profile” reported in Zwiftalizer is the graphics quality, irrespective of resolution - “ultra” is the highest. This is different from the “ultra 1440” resolution you can set in Zwift, which is the second highest after 4k UHD 2160 resolution. You can’t set the profile within Zwift, it’s chosen for you (unless you edit the config file). For shadow res. I assume that 2048 is the highest.
So my machine will happily run ultra profile / 2160 4K UHD resolution / 2048 shadow res. and display it on a 4K screen at 100+ FPS average (OK, drops to 70 FPS on a busy course such as New York). It looks very nice… 
Having a decent frame rate and the ultra profile activated makes the biggest difference to the experience, the resolution less so (at least between 1440 and 2160).
I’d sort of got used to the “low” profile at low resolution and jerky 10 FPS on my old laptop, but this is much more pleasant! One of the biggest advantages of the high-end PC is that the program loads in a few few seconds, rather than taking 3 or 4 minutes…