I have a tele MP thinline deluxe (p90s, maple fretboard). Problem is truss rod works fine up to 15th, near body joint. After that, it is just flat plateau. Result is buzz when high up the neck, as fretted note is "down in the valley" relative to final frets. The saddles and action now have to be pretty darn high to avoid that, and is more like 60's discount store model re: action. I would like the neck above 15 to be part of the relief. Was thinking perhaps shim to perhaps give some angle, but that may be from bad to worse. Any ideas, oh git gurus? BTW only camera I have is crap on phone, so pics aren't available...
It's normal for the truss rod to only really effect the frets before the 12th, and barely past that.

As for action at those higher frets, you're basically down to one of four things: the angle of the neck pocket isn't right (irrelevant as your guitar has a flat-angle bridge), the frets are not level (quite possible), your combination of neck relief and saddle height is not right (very likely), or you simply have impractical and unrealistic expectations of what 'low' action is (very likely, most people do).

So, check the relief. If you feel adjustments need to be made, try changing the relief and saddles together, rather than relying on just the truss rod. If you can't seem to make any advancement there, check that the frets are level; uneven frets are pretty common on MIC guitars.

I wouldn't advise you shim the neck, at least not until you've exhausted every other option. It can be a pretty hard and lengthy process to get the angle right, let alone do it in a way which is sturdy and won't murder the sound. I wouldn't rule out the need for a shim entirely, just don't jump right to that.
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MrFlibble has very good advice in his post. Those lower budget guitars are only going to get so good. I wouldn't go too crazy on the truss rod either. Even after adjusting the truss rod it could take the guitar some time (maybe a day) to adjust to the new tension. So adjust a bit and check the next day; repeat as necessary.