I do have both but am not sure when I can do a side-by-side test.

The MKH 40 have a ruler flat response but roll-off earlier in the bass end and quite sharply at 20kHz.

The 8040 go about 10Hz lower and extend up to 50kHz. The response is not as flat as the 40, but the designer says they are less coloured.

I will try them side-by-side on my Nagra VI when I get the chance.