nick how about 70-200mm f/2.8L (2.9lbs) version 1
for more extension, it's under 1300$ on amazon.
ur current lens,17-55mm f/2.8 IS, is about 1.5 pounds
check out
http://www.borrowlenses.com
u can rent the lens and try it out.
doesn't Andy have the 70-200mm f/4L?