I suggest that the 166 wheel alignment specs are WRONG and the 1.8mm toe-out each side is not required - I reckon they should be set parallel. But then, compared with Alfa Romeo, what would I know?
I agree with you, Alex; In fact, I think that Alfa themselves had a re-think on the 166 toe settings, as after a certain chassis number (can't remember which) they recommend revised toe settings of
Front -2.0mm total toe (-00 17')
Rear +3.0mm total toe (+00 25')
Compare these to the earlier settings of
Front -4.0mm total toe
Rear +5.5mm total toe
Then you can see that the toe values are more or less halved.
I reckon that Alfa used so much front toe-out on the initial set-up so that they could impress the motoring press with the quick turn-in.
Seems to have worked, hence the "thinks it's a hot hatch" headlines!
However, back in the real world, where we pay for our own tyres I would recommend using the later (-2.0mm, +3.0mm) settings, then you might get similar stability to your 164.
BTW, Jamie Porter of the Alfaworkshop also recommends the later settings.