They can handle just about as much power as you'll be able to throw at them. There are 9 second cars with the FRPP driveshafts. I'm running one in mine currently.
I doubt there would be big HP gains. It is a little less rotational weight, so there is some gain. It is the same with all rotational weight. Any weight you can loose is good, any rotational weight you can loose is better. It helps acceleration more than an HP increase.
OE in an '03 cobra is aluminum so no gains there. I'm running it because a buddy of mine had it cut to fit his earlier cobra that he was putting a 6 speed in. He ended up going to a Tremec, so since I was going solid axle it was the right length for me. If that makes sense.
I've switched back and forth between aluminum and steel on previous cars and while I couldn't feel a difference, I'm sure the gains, however slight, were there.
I'm currently running 4.30's and I have no vibrations at all well into tripple digit speeds. More than satisfied.