Generally mass production cruisers as the Catalina are less stiff then performance cruisers and performance cruisers less stiff then race boats. That is about the same to say that cruisers are less powerful than performance cruisers and this one less than racers.
For a given generation of boats, the cruisers always tend to be more stiff at low angle of heel because the most important person in our lives does not like to walk around on a deck that is not level.
My wife does not like the boat to heel above 10 degrees. Fortunately, our Catalina 40 is already doing 8 knots before this. If our Catalina 40 is heeling at 15 degrees, its because the boat is traveling at 10 knots.
Now looking at the polar diagram and heeling listing for the First 40, it is only downwind the First will not be 15-20 degrees doing 8 knots.
So you say the cruiser is less powerful at 8 knots??? Doesn't make sense, the Beneteau First 40 is lighter and less drag. So by definition, it will requires less power, not more power to run at 8 knots.
So since it requires less power, to go the same speed, but it heels more.. then by your definition of more power meaning more stiff, the First 40 must be less stiff.
Perhaps you would like to reformulate your definition of more power being more stiff.