I have an XRGB-2, works great with Amiga but it doesn't sample the entire overscan area (I don't think any "VGA box" does.)
It will look very ugly if you don't run it on your TFT's native res, so the XRGB-3 is probably better in your case.
I'm not sure whether there's an English speaking firmware for the XRGB-3, however.