You might also want to consider Canon's 70-300 L. Mine worked beautifully on my crop body camera on my trip to Tanzania last year. Although you cannot use Canon extenders (at least not throughout the entire focal length range), I understand that you can use Kenko extenders on this lens. I should note that I have not used extenders on this lens.
+1 on the 70-300L. While the 70-200 & the (current) 100-400L are great lenses, they are considerably larger and heavier than your previous tele. The 70-300L will greatly improve your IQ and marginally improve your reach, all in a compact and easy to carry package with the latest image stabilization. If you can wait for a 20% off refurb sale from Canon, they can be had for around $1100.
I love my 70-300mm L. It's a great portable lens, with fantastic IQ at any setting. 70-300mm range on my crop really works well for me.
Sure it's not f/2.8, but in low light I use primes anyway - usually faster than f/2.8 (eg f/1.4 - f/2).
(Note the recently released EF-S 55-250 mm STM is a great bang for the buck lens, better than any other 55-200 / 55-250 lenses before it).
Can you please outline some more details of what you'll be photographing with a tele/telezoom lens?