Is it effective on the highway? Maybe, but only then at the very top reaches of the speedo.
False. To be quite honest, I can feel it from anything over 55 to 75. At 70 the car begins to squat down and only gets more stable. If you will never be going on a freeway, go ahead and remove your wing all together. May your guardian angel be with you if you ever have to go more than 50 in a turn.
I really felt the difference on the track, especially at anything over 50. The car already has wonderful weight distribution (55/45) and doesn't like to get a loose rear in hard turns. I think it would be more loose if you didn't have that wing.
I don't have any problems with visibility on the wing either. If you can't see their headlights, then they are probably too close anyway as it only blocks your lowest portion of the hatch window.
Remember also that the suspension on the SRT6 is stiffer and will allow the back to slide easier than the Base and Limited models that will simply let the body roll a bit more to dig in.
INNERCOOL... I hope you find the right balance to your quandary.