Testing and experience have shown for years that for astro-imaging, Canon DSLRs work best at ISOs 800-1600. The DPr comparisons shows nothing has changed, although it may actually be slightly better at 3200 than other models -including the 80D, as noted in the reviews. Its just a shame that no one at Canon Corp. HQ in Tokyo listens to you. Sad! I'll be glad to give you a hundred bucks for your 5DIv and 600mm L lens since you're be dumping all that Canon crap on the classifieds.
Two EV is nothing. Try four, five, six. Compare to the 5D IV. The 5D IV kicks the crap out of the 6D II. Consider that an astrophotography stretch is a lot more than even a 6EV landscape stretch. So, 2EV...yeah. Let's be realistic here.
Well, there it is. That everlasting, undying, unwavering Canon loyalty, even when the facts have smacked you upside the head. I don't think I'll ever understand it...
Sure, Canon DSLRs do work best at 800-1600...and thats BECAUSE they have historically had such crappy low ISO performance! Let's not be naive here. I used 800 & 1600 on my 5D III, because had to. However it definitely limited my dynamic range. With a 5D IV, I'd be very happy using ISO 400, even 200 on the objects I needed more dynamic range for (i.e. Orion Nebula, Lagoon Nebula, Globs, many galaxies). With a D810, I'd be happy using ISO 100!
I'd definitely not be comfortable sitting at ISO3200 on a 6D II.
Of all the shadow pushers out there, of all the people who need more dynamic range, astrophotographers are the top, by a long shot. The kind of shadow pushing people do for landscapes pales in comparison to most astrophotography stretches.
If you are happy with ISO 800, then have at it. Personally, as an astrophotographer, I wouldn't touch the 6D II with a 50' pole. Not a chance, not when the 5D IV is several STOPS better, basically devoid of banding, and has measured dynamic range over 12 stops (which is well into the range of good CCD cameras...the best Sony ICX sensors have 12.5-12.8 stops).