First impressions: well, I am impressed :-)
So KEFs came today and I swapped out ELACs L/C/R for KEFs. I also got foam sound isolation pads for them. Not sure how much of a factor it is but thought I would give them a try.
After setting up the speakers, I ran YPAO, and my first surprise was that YPAO just nailed it. With the old mix of speakers YPAO never gave me good results, and I had to tinker with them afterwards but never felt fully satisfied. It was especially bad with subwoofers, and it was a pain to deal with. With all 5 matching speakers I did not have to make any changes after YPAO except for setting all of them to small with crossover at 80 Hz. I left crossover for Atmos speakers untouched. I did not change any other YPAO measurements.
I started to play Dolby Atmos and DTS:X trailers from youtube and got sucked into them as the sound was just so good. I am not professional, tone deaf, etc., and can't give you the same run down that professional reviewers can give you but I can tell that the sound was very well balanced, everything was in place, the bass was just right, not crazy but appropriately authoritative where needed, tight and well mannered. It was the first time I truly enjoyed bass from my system so I played and played trailers with nature sounds, especially storms because the way the thunder rumbled all around me was so true to life that I wanted to hear more and more of it. What was very pleasing was the clarity of all minute sounds where you could hear every clink, every creak. It made the sound very life-like. The sound from ELACs is nowhere near as detailed as the sound from KEFs. I also like that you can still hear all the details even at the lower volume when watching at night.
I played with them for several hours, not just trailers but also regular programming from Netflix, and I feel that they are already starting to break in because they sounded even better by the end of the night than they were in the middle of the day when I first set them up. And they were pretty amazing then.
All in all, I am very pleased with this upgrade. L/R are upright but C and side surrounds are on their side. Either way, they look just as nice as they sound. I am sold now on the idea of using the same speakers for all the channels. KEFs are very conducive to this due to their design as they can be used either vertically or horizontally with no ill effects. And the current price brings this proposition to the level where it could/should be seriously considered.
Are ELACs Debut 2.0 B6.2 bad speakers? Not at all. But they are just not in the same class as KEFs Q150. At the regular price of $600 for KEFs Q150, $300 ELACs were a better value. I know for sure that I would not have done what I did at the regular price. But $300 KEFs Q150 vs $300 ELACs Debut 2.0? There is no question here, you should go for KEFs. And build full surround system with them. You will love it!
Thanks everyone for taking time to respond and being supportive of this idea :-)!