Photons to Photos just published their dynamic range results for the Fujifilm X-T3.
According to their data measurement, there is no noticeable difference between the Fujifilm X-T3 and X-T2. The Fujifilm X-T3 has a little advantage at base ISO, especially thanks to the base ISO 160 and also at very high ISO.
We have seen from previous comparisons, that the Fujifilm X-T3 preserves more details at high ISO.
You can manipulate the chart here.
From my testing of the X-T3 at photokina, I can say that AF speed, face/eye AF and video are the real reason you might want to upgrade to Fujifilm X-T3.