As I posted back in Sept. I read somewhere, that you get about 40 lbs. of down force at 65 mph from the limited spoiler. I'm sure it will increase the faster you go, but I have no idea how much.
I agree with HDDP, I think the Whale tail is more for "looks", to distinguish it from the Limited. Because of its obvious size difference, I'm sure it offers more down force (at any speed) than the Limited spoiler does also. But I don't think it takes any "more" down force to keep an SRT stable at say 150 mph, than it does to keep a Limited stable at the same speed.
On the other hand, if HDDP says that his Limited likes to "Float" at 140mph, maybe he could benefit by trying out a SRT-6s rear spoiler.