Canon EF-S 18-200mm IS vs EF-S 17-55mm IS for Concert Photography – Paul De Bra:
I have used the 18-200IS and the expensive 17-55IS at low light concerts. I find that (in the 18-55 range) the 18-200IS does acceptably well at f/5.6 where the 17-55IS does at least as well at f/4. So the difference in usability for low light is essentially 1 stop. (The 17-55IS is also very good at f/2.8 but that often does not give enough DOF for my applications.)