You can get your Sigma 50-500mm from online shops for ~$700-800, which has already placed you into the high end of Canon "L" lenses.
If you can spend an additional $400, you will get a Canon 35-350mm f3.5-5.6 L USM - A truelly marvelous "L" lens that will give you all the pleasure of photography. The image quality is very sharp and you can grab those moment that no else can.
I had the 35-350mm L for 3 years. Had just sold it because I felt it could be replaced by my other lenses (I had 20-35/2.8L, 50/1.4, 85/1.8, 70-200/2.8L, and 300/4 LIS). The reality is that I regreted the moment I sold the lens. Soon, I found that although it duplicates my entire shooting range, its function is irreplaceable. Yes, it is not a 2.8 and not as sharp as any of the above lenses. But the image quality is very close. I bet you cannot find any differences on a 5x7 or 8x10 enlargement in a head-to-head comparison at 35-300mm range (350mm may be a little weak when compared to Canon's flagship 300/2.8 or 300/4). I found when I went travelling, I missed lots of shots when changing lenses (I even had two bodies, one D30, one A2).
The bottom line is, unless I stay at home and only take the lens I needed for shooting, I would rather bring the 35-350mm along with a 16/17-35 (or 15-30). Even if you have everything else, the 35-350mm L is still irreplaceable for travelling. I regret that I sold it and think I will get another one for myself again soon.
I have not used the Sigma 50-500mm therefore cannot comment on its quality and usefulness. If I were you, with the money invested in a Sigma 50-500, I would rather put in a little bit more and get a used 35-350mm L. It uses 72mm filter rather than the very uncommon 86mm for the Sigma. It is a USM with full-time manual function. And the most important - the image quality is unmatched by any super zoom out there.
That's my 2-cents. Hope it helps.
CHUN