I have read on test patch forum that scawen is now able to change anytime in about 90 sec the handicaps amount added with W9 for each car he want (particulary for TBO and GTR classes).

This a good thing.
For him.

Because if you are racing in a championship and developing a setup in, for example, a 2 weeks time (if the champ has a race each 2 weeks), and 2 hours before the race, Scawen change the handicaps, the set that you have developed with hard work for 2 weeks will be mess up.

Imho, this problem can be solved in that way:

creating a webpage on lfsworld, liveforspeed.net, or directly in-game where at anytime players can check actual handicaps and planned handicaps changes for the next 7 days.
Each change should be pre-advertized here 7 days before, leaving the time to racers for changing their sets.

Imho that solution will solve the problem

I really don't think it's going to be a regular occurence. Also, the magnitude of each change is likely to be smaller with each change, as the restrictions even out the car classes.

Nah, part of the fun would be not knowing how the cars are going to be balanced. Your two weeks work won't be wasted, but some fine tuning might be needed. In real life the organiser is able to change ballast/restrictor requirements at short notice if they wanted to (though they won't change restrictors at very short notice due to the need to reconfigure ECUs etc, a factor we don't have to worry about thankfully).

The thing that worries me is that the performance difference in each class might be ironed out completely, but with certain cars (FXR for example) being much easier to drive than the others.