As far as I'm concerned, lenses are never overkill....

It's not just the IS that's an issue with better lenses, it's focal speed and accuracy. The L is not mandatory either. I've compared the sigma offering to my L on a 1ds MkIII(not mine) in a tet bed situation, and found very minimalistic differences.

As far as IS being useful: what are you doing that it improves a shot that is not already motion blurred? Hand holding down into the 1/100th shutter range is fairly simple, and below that blurr becomes an issue from simple motion. I've just never been in a situation, save one wedding in particular, where IS did anything for me, and that was a still shot of a bride and groom indoors. It did allow me to close the lens down a bit, and compensated for my flash dying as I snapped it, but they where stock still. I just don't find it useful in any sports activities, especially in a poorly lit stadium or rodeo arena, where the large element and lower aperture come in so handy. It's also very easy to drop a 1.4 or 2.0 tele onto a 70-200 and get a nice ~300-400 f4 or f5.6 lens.

In honesty, if I was the OP, I would never have bought the 50d. The 30 is well MORE than capable of what he's doing. I shot for nearly two years with a rebel with this L hanging off the front, and took some truly good quality(we wont talk composition.....purely a equipment discussion LOL) shots with it. The MkII simply fell into what I needed from a speed and performance standpoint. I come from an old school of photography where a body was simply needed to be capable of the minimum of what you needed, everything else is LENS. Equipment is about 10% of the shot, the other 90% is the shooter.