<p>They aren't too heavy if you have a reason for them, but for me, anyhow, I wouldn't bother to upgrade, necessarily. I did exactly what you are talking about with the 16-35/f2.8 which I took in place of the 17-40. My reasoning was that the extra speed would be better for star/milky way photography, but it turns out that the 16-35 has horrible coma wide open. I ended up selling the lens, though I liked it, because the 17-40 is also excellent, and a heck of a lot lighter.<br>
Likewise, how often are you shooting at f4 on the 70-200 now? That's a decision I weighed too, only I have the super-light, & cheaper non-IS f4L. I determined that the only think (ignoring IS) that the 2.8 would get me was more flexibility with extenders, but that gives you a lens at 400/5.6 if you use the 2x extender. I decided to get the 400/5.6 prime instead, which is a GORGEOUS lens, especially for the weight. If I'm shooting wildlife it comes along, but if I'm sticking to wide-angle or medium tele, my bag is super light. I just did a strenuous 2 week trek with a 6D, 17-40 & 70-200 on my back, and I still had a little room left over for snacks. :-) <br>
Anyway, good luck deciding, it's a fun problem to have!<br>
<a href="http://www.annedirkse.com">Anne Dirkse</a></p>
