I use a fp L with a Cambo Actus G. I have also used a Nikon Z7 and Panasonic S1R as my digital "backs" in the past.
How do you like using the camera in this role?
It works well. It may be one of the few use cases where the camera's design compromises don't matter much at all. For landscape and architecture it suits me quite well. The screen does not tilt which is tricky in some setups and the focus peaking lines are a little thick. Battery life is not a strong point. Otherwise the files are fine. DXO PL5 support should be getting close now, too.
Would you buy it again or go a different way?
I have made a decision to consolidate everything to the Sony A1 system for all my shooting from now on i.e. adding air shows to the landscape and architecture mentioned above.. Once I get the appropriate Sony lens plate from Cambo I may no longer need the fp L. However I can live with its compromises and it is a very compact package, especially when paired with a small Nikkor SW or other compact LF lens. On the other hand having one set of customised camera controls that you can operate automatically and intuitively is important, which the A1 will let me do.
On the other, other hand, I can carry the whole setup in a small bag like the Thinktank Retrospective 5. But if I add a RZ67 lens on the front and a bigger camera on the back then the whole package is quite a bit bulkier, in relative terms. So the fp L does help me carry around a rather svelte piece of highly versatile and capable kit.
The Sigma i lenses make it a portable general purpose camera, but then the design compromises may rear their head again i.e. electronic shutter only, no stabilisation, no grip, battery life, poor continuous AF, no screen tilt etc. etc.
How wide an image (sensor mm) can you get with:
Pentax 645A 35mm
Rob de Loe has commented on a thread here or on DPR about the useable image circle on this lens (which I once owned). I think on a 36mm x 24mm sensor you can shift about 12mm each side (i.e. laterally to the sensor), but I wouldn't go past that. Excellent in the middle though if you don't plan to shift for stitching purposes. I may get another one to use as a 35mm lens that can be tilted and can be used for a small amount of rise or fall at the same time. For that it would be excellent, once you have obtained Cambo's P645 lens plate.
Mamiya 55mm. (I have a late P67 55mm, but it is a chunk).
I use a RZ67 50mm ULD lens and it is excellent, Even when shifted 20mm each side it is still very good at the extreme edges. That gives me very large images that are a little over 21,000 pixels wide and not quite that much tall. So I am very happy now in the IQ department.
Any random thoughts/pearls of wisdom on the concept and execution?
Lack of sensor cleaning may become an issue over time, especially with the Actus. I think that in combination with some of the "i" lenses from Sigma (20mm, 45mm, 65mm) it could make an excellent and very lightweight kit for landscape photography when hiking. Don't forget to pack an extra few dozen batteries though. And you might need a small tripod.
PS. Anyone use the LVF-11?
Tempted, because in strong light the non tilt-able LCD can be a struggle, but I haven't been shooting much lately and so haven't felt the need. Maybe a Hoodman Loupe i.e. optical, but of course you can't tilt it.
Hope that helps.