I took the 777-300 SFO to LHR and then back again in June. Same fare situation as you, so I bought a W fare, hoping to upgrade with GPUs, which did not go through. Sat in E+ middle seat of the 3 seats on the right side on the way to LHR (a K seat, I think); sat in E+ aisle of the 3 seats on the left side on the way back to SFO (a C seat).
Even with the 10-across seats, I felt fine, slept fine, had enough room to read, watch movies, eat, etc. It helped that I had nice, relatively small-sized people sitting by me (I'm medium height and thin, as well). I didn't notice that the newer seats were thinner or had less of a cushion. The flight times worked for my schedule. The FAs were nice and the service was fine for economy.
I'm going again next month on the 777-200 and am waitlisted for an upgrade to business, which I actually think will go through. Maybe I'll get on of the few planes with the new Polaris seats, rather than the 8-across sUA-style! This flight was picked based on the schedule I need for that day. We'll see . . .
Basically - I decided that the schedule timing and nonstop flight were more important than the flat bed for that enormous price. Good luck - and, please let us know what you end up doing and how it works out!
JK-SFO