legally speaking it is too. So if you get stopped while driving someone elses illegal car (eg bald tyres) its you who get the points and fine not the owner.
your 30-40 MPH test is one thing, but the court would argue what happens if you have to stop from 60, 70 or faster? What happens if you have to stop with a greater load in the car or different weather conditions....
An engineer's report that your wheel/tyre combo was safe would be about the only thing I can see being admissible