What you just wrote is why refrain from Sigma. Nothing against 3rd party, hell I'd even buy a Rokinon since it gets the job done without grief. IIRC Dustin Abbot or someone did a full on review/comparison of the 150-600 and even he concluded that Sigma missed focus more than the Tamron.
Hey there's a reason they sell another component (dock) that is supposed to fix what shouldn't be broke in the first place.
Again I hear you Within 1 meter of where I type are 8 (eight) Rokinons and woohoo soon to add the Rok for EOS-M mount. Hey, just remember FUN!
Don't let all this internet/forum mumbo jumbo take the little kid out of it You know the world is changing-and quickly. There might come a time when all
this digital stuff just don't matter.