5milesbaby,
As you are an ATCO & mention the A320 (my current type) and it's climb perf, or rather lack of, I offer my observances.
The V2500 engined 320's climb better at altitude than those with
the CFM56 engine
Climb performance is satisfactory up through the mid-levels, but >1000fpm is out of the question above FL 250. This is just below the point at which we transfer from flying IAS and go onto Mach.
I have tried high-speeding the thing onto the Mach No's as soon as poss (which usually works in older & heavier jets) and this can offer temporary respite up to approx FL300; from there on its a long flog up to "Optimum FL".
My own gut feeling is that the engines, particularly the CFM56, don't like high Mach Nos and lose their propulsive efficiency, but as my old prop notes from ATPL days are stored up in the roof, I can't offer any clever scientific reasoning for stating this!
Incidentally, I have experienced these characteristics once before in my career, on the early MD80's, which climbed well to FL250 and then dropped off markedly, much as the 320 does.