Thanks for your replies,
I'd like to compare with RAW, but I'm a Mac user, so I guess I'll have to wait until Sony brings out the software...
The Jpegs and Tiffs are, on closer scrutiny, not exactly identical. In deep schadows Jpeg seems to throw in a few more lighter pixels than Tiff does. Apart from this, however, differences are very close to zero. At sharp edges, where you would expect Jpeg artefacts to show up, I have yet to find a single pixel to be 'out of line'.