So is the consensus that these 9micron backs ~22mp are great at base ISO which is likely 50 or 100 and then really not so good at 400?
My application is location portraiture and sometimes you need ISO 400 to drag the background up even though you are lighting the portrait with strobe, you need the fill level.
For example a likely exposure is f8 1/15 at 400ISO- just enough drag to keep the background lively.
I will also just shoot ambient at 100Iso, say 1/2second on tripod.
Are these likely to be fails on a fatback?
Should I be saving my scant pennies for a back that can do decent 400? If so what is likely to be south of 10k? considering i like wides too- I know, make it impossible.
A P25+ at ISO400 has a moderate amount of grain, but processed in Capture One v7 it can be, in my opinion, beautiful. It could be had for significantly less than 10k (back only) and probably under 10k in a kit that would suit the needs you describe.
Asking for other people's opinions on what will be "usable" in an ISO range is to some extent a fool's errand as everyone has very different definitions of "usable" and different aesthetics.
I'd get in touch with a local dealer and ask for some raw files, or better yet, to shoot your own (in the situations that are typical for you) and process them yourself and see what works for you and what doesn't.
But yes, in
general, the fat-pixel backs were earlier generation models. And in general those earlier models did not do great at higher ISOs. Advances in raw processing (like Capture One v7) mean that someone who used the back most recently years ago will likely have an out-of-date opinion about what ISOs they could make work for them.
Many of the backs on that list would have no problem with 1 sec at ISO100. Its the ISO400 question that would challenge many/most of those backs.