You would need a powered one, because the regular FEThead (and Cloudlifter) is powered by the 48V phantom power supply, which, or course, your guitar rig does not supply. It will still be a low impedance signal, just louder. The SM57 (and the 545SD) have about 1.7mV output with low impedance (around 200 Ohms), A guitar pickup is a hi impedance device (typically 7-12kOhms), and it's an unbalanced signal (the ground and shield are combined). In Hi Z mode, the 545SD is 17mV, so significantly stronger.
Why would you spend £50 to >£100 on a step up device when you can just get a mic for about the same price and have something that you can actually use for other stuff too?
Dynamic all-round microphone Polar pattern: Cardioid, 50 - 15.000 Hz, Versatile, high-quality microphone with SM57®-like sound characteristics, Lockable, Silent magnetic on/off switch, Silver housing, For instrument and speech recording, Output...
£122.00
www.thomann.de
Classic Unidyne instrument mic features selectable dual-impedance operation with a silent on/off switch.
www.shure.com
I'm still a bit confused about how you'll use it. Are you going through a guitar amp? Using a looper is not a permanent way to store stuff. It's like using a pipe wrench to pound a nail. It'll work, but its the wrong tool for the job.
I know that the 545SD in hi Z mode will work, because I have an old Shure Unidyne B 515 from about 1968 that is hi Z, and it worked, although going through a guitar amp it was really susceptible to feedback! I hit my H2O chorus and my Soul Food pedals and they obviously changed the sound. Using a waa-waa gave a nice annoying squeal!
Why do I have an old hi Z mic? Back "in the day", we used a Kustom PA head that used hi Z inputs, so those were the mics we bought. It was basically like a 4 channel guitar amp with "full range" speakers. Sound wasn't great, but what did we know back then?