EF 28-135 Good enough for FF?

Macabron wrote:

I would go for the full frame . Add a 50 mm f1.8 and that will give you a sharper (at f2.8 and above) image and a low light lens. After using your 28-135 for a while you can decide if you want to upgrade.

I would suggest this only if low light photography and ultrashallow depth of field are really his preferences/priorities.

The major counterargument is that two lenses the OP already owns are for crop cameras only, and he indicated that he does not have/does not want to spend money on new lenses.

While the EF-S 15-85mm IS can somewhat replaced by the EF 28-135 IS (albeit with loss of the 24-28 mm equivalent focal length range), this is not the case for his Tamron 60mm f2. The Tamron 60mm f2 can be used on crop cameras only, (1).

For this reason, replacing the 40D with a 6dII only makes sense if the Tamron Macro sits in his drawer unused. However, the OP indicated that he uses it for portrait and macro. While the first use can be somewhat covered by the cheap 50mm f1.8 suggested above (not fully, as the equivalent focal length of the Tamron is ~100 mm) this is not the case with macro.

For this reason, if I where in his shoes, I would get the 80D - unless low light and shallow DOF are fundamentally important. Then he would either need to 1. sell the 2 crop lenses to contribute to funds for full frame lenses or 2. keep his 40D as a second body for use with the macro.