Don't be too certain that you will get much more coverage by stopping it right down - most 360mm plasmats have mechanical vignetting at the limitation of the image circle. For example, I have a 360mm Apo Sironar which actually does not cover 11x14 because the filter ring vignettes when you stop it right down. I'd be careful to check the exact lens you have in mind before forking out for it. I had a Schneider Apo Symmar 360 which had mechanical vignetting at the spec'd image circle.
I guess the 355 G-Claron would be a better choice since this lens appears to be well respected for 7 x 17 and allows movements.