I just found this test (see link below). I didn't do the test, I don't have any additional info on the actual test and I'm not sure if it's been posted here before? Quite interesting anyway.

Best,
-terje

Untitled on Vimeo

Not a big fan of technical "bake off" videos but this one provided some useful results.

They perform right in line with their price points.... no big mystery, nor surprising.

you get what you pay for. I think alot of people find the DSLR's amazing in lowlight because fast primes are very inexpensive in the still world. Truth be told the DSLR's dont perform well at high ISO's. It's just that with a Canon 85mm L series F1.2, you can be shooting in in no lighting situations @ ISO 500 ish. To do the same with a 35mm PL lenses you need to spend $400-$500 to rent a set of Super speeds for the day. The canons make fast glass accessible and convenient.