I don't know if anyone else saw that I asked Amy what the current capacity of the welded studs are, she said her supplier has them rated at 300amps, I'm not sure if that's realistic, but the more I think about it, the more I think the contact area can probably carry a lot more than we think it can as long as the oxide is cleaned off. Think about the contract area on high powered relays, it's not huge.
Harking back to my physics days:
Resistance =ρl/a (where ρ is resistivity, l is length, a is area)
so on a contact patch l(length) is very small compared to a length of cable, so should be able to carry much more current without heating up due to lower resistance. .
Before I came across this thread I had mulled over what you are attempting with washers, and decided that unless I could find a washer that is soft enough, there is potential for the weld causing an unparallel washer therfore reducing the mating surface even more, especially when using solid busbars where adjacent studs may be in vastly different planes from one another due to cell assembly and stud placement differences. (On my original eve 280 cells with the tapped terminals, a swipe with a hone showed that adjacent factory terminals were unparallel to the point that a busbar would only be sitting on about 30% of the total area) In the end I decided that using cable with lugs was the best way to maximize contact area, and silicon grease to protect from corrosion. We will see how it goes when I get them in service.