Impressive numbers. Hot Rod just dyno'd one right off the lot. 405/405 RWHP/RWTQ. They also mention a "strange" dip in torque right after 6000 RPM - they suspect some sort of torque management or attempt to keep it under the Vette.
As far as comparisons to a 4th gen - I would LOVE for my 5th gen to weigh less but there is no comparison. My 4th generation, even with sub frame connectors and strut tower brace felt like a limp noodle compared to my 5th. With IRS on top of that it is hard to argue it should weigh 3200 pounds because the 4th gen did. Again I would love it to but it is not that type of car.