I think the infrastructure improvements are necessary in the long-term.
In terms of trying to get the fastest service possible, it's a question of being able to timetable it so that other services on the line are passed where they're stationary on the loops.
Thurso and Wick are musts; aside from that, you pretty much need to stop at Helmsdale, Brora, Tain, Invergordon and Dingwall. Lairg is away from the A9 corridor so should probably get a call; the same applies to Ardgay. Alness doesn't have a passing loop, but it's the largest population centre in Easter Ross. Golspie is the largest place between Tain and Thurso on the line, so that gets a call too despite its lack of a passing loop.
Muir of Ord (a small town, with a passing loop, fairly close to Inverness) and Forsinard (the last passing loop on the line) wouldn't be at the top of the list, but ultimately calling there might be needed. Rogart also has a loop but is usually a request stop (though it gets more passengers than Georgemas). I think we can drop Fearn; Beauly and Conon Bridge are served by the Kyle services and aren't far from Inverness, so can be omitted from the one fast train per day; Dunrobin Castle maybe in summertime, or Evanton or Halkirk when they get opened (but that would signify infrastructure improvements anyway).
It's then a matter of timetabling it so that you can get non-stop passes at places like Muir, Rogart, Forsinard and maybe Lairg. If it helps with making time and is feasible, then maybe you can drop calling at Ardgay and Helmsdale too.
In any case, loops at Lentran and Kinbrace (and possibly at Evanton, Edderton and Halkirk while we're at it) are needed anyway. After that, not only can you go hourly to Tain, but there's the distinct possibility of faster services yet.
