I would say that current ratings are not overly "pessimistic" (except possibly for cars with unusual drivetrains), but rather that they're now "realistic" for average drivers instead of "too optimistic" as they were before. The previous standards assumed "highway speed" meant a maximum of about 55 mph, for example, when the vast majority of people drive much faster than that.
Strictly going by cars that I've owned and tracked MPG for more than a few thousand miles:
1988 Chevy Sprint Metro
Original Rating: 54/58MPG
"Revised" Rating: 44/51MPG
My MPG: Consistently 53MPG over 26,000 miles. Best tank was 57MPG, worst 46MPG. Arguably an even split between old rating and new. However, this car was made before ethanol in gas was commonplace, so that brings down MPG with E10 gas, which I used almost exclusively. Also, it was 24-25 years old when I owned it. I'm sure when it was brand new with real E0 gasoline, it hit its original rating.
1996 Volvo 850 Wagon
Original Rating: 20/29MPG
"Revised" Rating: 18/26MPG
My MPG: Average of 26MPG combined city/highway over 13,000 miles. Around 23-24MPG in town, and 30MPG on the highway. Pretty close to the original rating, far beyond the "revised" rating. 160,000+ miles on the car at the time, still running great.
1999 Chevy Metro
Original Rating: 41/47MPG
"Revised" Rating: 34/42MPG
My MPG: Average of 45MPG over 12,000 miles. Includes both in-town and 70-75MPH interstate madness. Not much variation between city/highway that I've noticed. Right in the middle-high end of the original rating, leagues ahead of the "revised rating". 158,000 miles at the moment, engine is a little weak and might be losing some compression (testing later this week).
1992 Buick Roadmaster Wagon
Original Rating: 16/25MPG
"Revised" Rating: 15/23MPG
My MPG: Only have put about 2,500 miles on it, but in that time have averaged a total of 18MPG. Does about 20-22MPG highway (at 75-80MPH, haven't really done any more sensible highway driving with it) at the moment, and 16MPG in-town. 160,900 miles at the moment, running a little rough at the moment. Here the ratings are pretty close, but it's really hard to make a call which is more accurate until it's been driven more. Also every single mile has been with AC on, because the car is a greenhouse, so that skews it down some.
Overall, yes, in my experience the "revised" ratings are rather pessimistic, especially the "city" rating. And the newest of these cars is 15 years old. I'm sure straight off the assembly line all of these cars did even better.