ColourOfSpring - 12:56 pm on Mar 22, 2013 (gmt 0)
I feel sorry for anyone having to adjust a large, existing site to be responsive. Designing Responsive is like designing three websites at once.
I'm not sure of the effect it has on SERPs, just whatever you do, don't be like NASCAR and design with tablet users in mind first, then mobile, THEN desktop.
There's always been this kind of developer peer pressure to get your site "made for mobile" going back as far as the late 90s. I think if you keep your original page designs simple, you shouldn't have to make radical changes for a smartphone version, if make a version at all (depends how simple the layout is).