My impression is it's not that hard in many Gr4 cars. Huracan, Viper, Corvette, Cayman, Silvia, etc.
I agree with your list of Gr4's with the caveat of not that hard with manual transmission, it's harder with automatic. I've been grinding the stock Gr4's the past couple weeks, mostly on hard and on MT. All cars were stock suspension, gearbox and aero, no tuning at all on racing medium tires.
Hard mode MT dry race winners:
1. Huracan, one of the strongest Gr4, one stop FM1/2 with a little coasting to manage the 4 lap stint then blitz the final three on FM1
2. Cayman, one stop FM1 with standard short shifting fuel savings, not quite the pace of the Huracan but strong.
3. Megane Trophy '11, one stop FM1 no fuel concerns, not the best top speed but excellent apex rotation and exit speeds, very fun!
4. SLS AMG, one of my new favorites, can get 4 laps on FM2 for a one stop or 2 stop on FM1 and win either way. Feels like a MR.
5. Corvette, two stop only, needs fuel savings on FM1 to make 3 laps, best to do 3-2-2, feels clunky like the Viper.
6. V8 Vantage, another 3-2-2 two stop and a not so easy dry race winner.
7. Viper, it is what it is.
Many other winners in wet races but those are the dry race winners so far. I'd put dry winning percentage at around 60%.
Hard mode AT dry race winners:
1. Huracan, the one stop becomes a two stop on FM1 with a lot of coasting to get the first 3 laps. Very similar to the Corvette on MT.
2. Megane Trophy '11, can one stop with AT, savings needed for the 4 lap but can blitz 3 on FM1.
I've only tested a few with the automatic and I'm sure there's more winners, but the margin of error is much smaller with AT. Fuel economy is king and dropping to say FM3 to make 3 laps is a killer.