The greater the zoom ratio,the greater the hit in IQ and lens speed on the average. Hint.
The Canon 100-400 is legendary Pro Glass and doubly so with the Version 2 and it's innate ability to better support an extender for long reach,on top of the very much improved relative IQ,across the zoom ratio and aperture sizings. The IS is also a real improvement too. Hint.
Someone who "knows" as "much" as you,is always going to be better served by asking questions...than giving "answers".
HINT..................
