Hi Tony,
To a large extent, anything above the 14th fret is beyond the proper range of the guitar. So while it is possible to play up the dusty end, you are pushing the limits once you get that far. A good rule of thumb is to assume that the highest frets are there for decoration until proven otherwise.
First of all, the tone will get a lot thinner once you get to that part of the neck. Due to the way guitars work. Heavier gauge strings could improve things, but there will always be a some loss in that area. And heavier strings will make the intonation problems worse.
If you do play a lot in that part of the neck you might compensate by looking at your amp and FX. Maybe try a treble booster and/or a compressor.
With the intonation, there are a few possibilities to consider. But it is unlikely that a set-up alone will be sufficient to solve the problem.
Starting with the worst case, it is possible that the higher frets are in the wrong place. In that part of the neck half a millimetre makes a big difference. Mass produced fretboards are usually made with a multiple blade table saw. All the slots are cut in a single pass, and there is no guarantee off accuracy.
The next worst case is that the top of the fretboard is deliberately ramped away from the strings once you reach the neck/body join. Effectively increasing the action progressively as you get to the higher frets. And as the action increases, you have to stretch the string further to fret it, and the intonation gets progressively sharper. Manufacturers do this because it saves a lot of time in final set up. By getting those frets out of the way it's a lot easier to get a reasonable set up for the first octave very quickly.
Next is that the neck has developed a hump where it joins the body. That has a similar result to the ramped fretboard.
Finally, the width of the fret itself can cause problems. If it isn't properly crowned it can move the intonation sharp or flat. And if the crown is worn flat it will move the intonation sharp.
Even a good set up won't address any of these issues. So you need a skilled and experienced luthier, and it won't be cheap. In fact, I would go as far as saying that if someone offers to do it cheap they either don't know what they are doing, or they are doing you a big favour. Make sure it is the latter.
If the frets are totally in the wrong place the most likely solution is a replacement fretboard properly fretted. There is an outside chance that the frets are wide enough to be re-profiled with a tall crown on the nut side. Very difficult even for someone very skilled in fret work, but a possibility if they have a Plek machine as that uses a special cutter to shape the crown. They can put your guitar on the machine and it will measure whether or not your guitar can be made to intonate accurately for the full scale.
If the fretting is accurate, but there is a ramp or hump, it is possible to remove the frets, level the fretboard, and refret with new wire. There are some manufacturers who offer guitars with narrower fretwire above the 12th fret. Any decent luthier would be able to offer the same for you. I've written a tool to calculate fret positions dividing the Octave into any number of frets, for a future project I plan to build a guitar fretted for 1/4 tones (24 frets per octave) using fat fret wire for the semitones and skinny wire for the quartertones in between (as an aid to navigation).
Basically, a decent luthier will be able to get your guitar eminently playable above the 12th fret. But it will take a lot of skilled work.
If playing at the dusty end is important to your music you might be better off searching out a guitar that is already very accurately fretted in that area and then considering getting further work done by an expert to fine tune it.
Good luck getting it sorted, and I hope that helps
Andy
