grats buddy suddenly why I use the word brick becomes clear
and in answer to you question................ lord it's tough

if you aren't going to miss the low light of f2.8 Russ if you need reach more than that........ then the DO is worth the upgrade. My DO handles better than the 300 is leastways for me on a 1 series body. The 300 2.8 is up there for sharps if the DO is lesser it's hard to see !!
Even though it's a mark two lens and you have those big 1 series batteries you want strength in light to really make the 2 X sing mate,i'm not really technical but you need that here . F8 can be limitng when it's grey here. As with the 300 IQ takes a slight hit over the 1.4 , with the 2X but it's a very capable combo . Ha 800mm on a full frame body that handles so well is VERY useful

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Russ bar the
second hand market look for black friday deals end of november................... I managed a 3 year canon warrenty a grand or so off then list price and a couple of years interest free to ease the pain,which is all but sorted. It was a game buying it Russ but three years of no worry and 20 or so percent off made it a good choice for me. I part exed the 300 The facet you loose is a 300 lens at 2.8 wide open which with crepuscular beasties is what I miss. I don't think I can find another negative Russ. I'd gladly ping ya a RAW or two at the weekend if it helps,
I think the boken is slightly different in some stronger backlit situations ie the,specular hilights in particular with the DO tech to the more conventional design . From what I know of what you shoot Russ, I think this might work well for you. what you shoot it's quite different to me in some ways and funnily enough I've just said it's a bit of a niche lens,but having come a similar route I can't find any real negatives bar more blummiin money