Some updates.
Went to Nissan last friday with my new pair of Falkens and a sad story.
They were very helpful this time. The head of workshop had just arrived from holiday and supervised the check of my car by himself. They even doublechecked the alignment in another workshop to make sure to get the same numbers. And they didn't charge for the alignment at all, just for tire change. Perhaps felt a little guilty for the previous time
We have had a couple of really wet days here now, and I have to say that my Z feels like new again
Who knows, was it alignment problem or just a lousy pair of T1R-s.