I was curious if LF shooters use 400 speed film as much as MF or 35mm shooters do?
Sure. But there are precious few ISO 400 options in LF, so people also do nutty things like push the holy hell out of slower films. The extra grain makes
very little difference when you contact print or make minor enlargements. in fact the extra grain might actually be beneficial; I've seen cases where in my opinion it is. And I have seen 100 speed films pushed many stops (2 or 3) that nevertheless yield tonally smooth images. Personally, I've pushed hp5+ a stop or two in 5x7 format and really liked the results.
Better sense, I don't think I've gone to 3200 yet, in LF, but why not. I'd recommend trying hp5+. I bet you'll be surprised how smooth the print is.
Some time back I made some simple arguments about the relationship between format size and tonality that may (or may not!) interest you:
(there was a url link here which no longer exists)
These arguments suggest that you can push to your heart's content in LF before you start to see a lot of grain, assuming of course that you will contact print or make minor enlargements.
Note also that there is a Fuji instant film, fp3000b45, that is an honest to goodness ISO 3000 LF film yielding very smooth images.