I picked this lens up at release, it is a beauty with both extenders as seen above. I have used it with the R3, R1 and R5 II. To me it is just about perfect for action with pets too. Such a fun lens.
Bare in mind all of these are with the 1.4ext and shot through very thick 'viewing glass' at Colchester Zoo, yes there is a bit of refraction but I am still impressed with the quality of the shots.
I know I know - more racing shots ;-) hey it's what I do! set to show the wide variety of series I cover in a single meeting, it's between 400 - 500 cars / drivers that I have to cover over the weekend - I try to get a minimum of 10-15 shots of each and every one at varying points around the circuit.
Bit photo heavy sorry. All with R1 RF100-300 2.8 and 1.4 ext.
Hadn't used the 2x extender very much so thought I would give it a test after the firmware update on the R1 - Banham Zoo. bare in mind through mesh fencing (so impressed with this set up)