...exactly how representative is it of 8x10?

I really would like to jump into a LF system for landscapes. 8x10 (and actually 4x10) is where I am fairly certain I'd like to end up, but I'm having difficulty getting clear answers on some threshold questions. Image quality ("resolution") is really paramount to me; I regularly print at 6-8 feet. The siren song of this 8x10 vs. Phase One comparison is haunting.

1. My understanding is that

The rule of thumb is that resolution measured in line pairs/mm is limited by diffraction to 1500/f ratio. At f/1, the limit is 1500 lp/mm. At f/22 the limit is 68 lp/mm. And so on.
(from this thread).

Since larger formats generally require smaller apertures - especially for sufficient DOF - how representative will 4x5 film be of 4x10 or 8x10? Velvia 50 is spec'd around 160 lp/mm, so using the rough 1500 rule you'd have to be shooting at f/8 to avoid the diffraction limit. That is wide open for most lenses...at best one stop down. At what point does the smaller format actually yield technically better results due to these coinciding variables?

My frustration lies in the upfront cost of an 8x10 system. Cameras are fairly reasonably priced but the good modern lenses run around $5000 when you can find them (ie., the 150mm Schneider f/5.6). Getting real exposed film in my hands to answer these questions seems like it will cost north of $10,000. I'd have no real issue starting with 4x5 if I had a better read on how representative the comparison would be. (I'd also have no problem just diving into 8x10 if I could get better information up front.) Does image quality scale linearly with square inches? Or do these other factors reduce the benefit of increased film size?

Joe