Ike was just a Cat 2 at landfall, but it was huge. Devastation in Galveston. This one is very large, too. We shall see . . .
True. It was also no Katrina.
If Issac can get organized over the next twelve hours or so (which seems likely), then it will probably slowly strengthen; I'd say it has a 5% chance of reaching the kind of intensity that Katrina had at landfall.
Ike was Cat 4 when it hit Cuba, and was a high Cat 2, nearly a Cat 3, when it hit the U.S. And, as you note, it was HUGE! The surge was more typical of a Cat 4 storm, and much of the damage was from the surge.
It remains to be seen whether Isaac will be that strong at U.S. landfall, but we know Ike was stronger in the early track.