Why?
Are the L chips cheaper (I suspect not)
They'd probably work.
"L" stands for low voltage. If EFI does not lower voltage it could unnecessarily overheat.
No.
Just because they are 1.35V rated does not mean they wouldn't work perfectly still at 1.5V.
I said it could. I see no point of getting low voltage RAM for workstation. 0.15v is not much, but still it depends on RAM BGA chips. I would not want to risk, probably no one have tried L RAM in Mac Pro.