Hi Valvebounce,
In order to make sure I have downloaded 550EX manual and even checked on some websites and 550EX is e-TTL v1 compatible. That basicly means older metering compatiblity, no in camera options, camera gives flash less information so that it may make decision how much flash its needed. Dont get me wrong I used to own 550EX and back in the day it was stellar flash. It still is today, but more in a manual brick kind of way that can output a mountain of power. It is reliable and well made and if that is purpose yaaay
ETTL and ETTL2 is decided by Type A, Type B and Type C camera compatibility. All cameras from 2012 and onwards are Type A and include 5 groups (a,b,c,d,e) and RT functionality. Ones from 2004 or somewhere around that time till 2012 are Type B (30D, 40D, 50D) and are still E-TTL2, but only have 3 groups and optical master slave. Type C are all cameras prior to that like EOS V and alike and are compatible with E-TTL version 1.
All EOS cameras are backward compatible, so 550EX will work on even newest camera, just wont measure light according to latest standard. Flash compensation should work from camera, due to nature of FC, but any other change wont.
I own YN600EX-II RT and i love it. I do have to moan about rubber gasket that is used to lock flash to camera ... those are such a flimsy design and get lost sooo easily. But other then that ... stellar flash.