I must’ve just thrown Skin the Gerbil close to a hundred times. I made a few observations, but the thing that stuck out the most was: aside from precession and tilt, ALL of the test bearings performed this particular trick very similarly for me. The amount that it “mattered” was really minor, especially for those of us who yoyo (no matter how passionately) as a hobby and not for competition.
Next time anyone is tempted to chime in on a bearing conversation with something that “sounds right” or “seems intuitive”, I would encourage them to take an hour and a handful of different bearings and just throw the same trick or two over and over again, occasionally swapping out bearings. You’re not going to notice an important difference in either the string rubbing (as popular mythology claims) or in the response engaging (as I’ve claimed in this very thread).
Anyhow, onward:
The main test setup was a C3 Krown with recessed red gasket maker response, and a slightly used (put it on this morning in the Krown which was today’s throw) YYF 100% poly string. Specifically it was one of their tourniquet (extra-long) strings cut down for normal 1A. I intentionally wanted standard poly that wasn’t brand new.
The bearings were: OD 10-ball, Dif KK, CBC CT, Buddha Whipple, Twisted Trifecta. I threw between 5 and 10 Skin the Gerbils (didn’t keep super careful track but closer to 10) after each bearing swap, and swapped them back in at least twice each.
The main test was simply: “do Skin the Gerbil with the best alignment possible, but with the so-so technique of allowing the string layers to slide against one-another” (with better technique, this sliding is minimized, but I wanted to test the string-on-string friction properly).
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It didn’t take long to notice that the flat bearing caused the yoyo to be more prone to tilt and precession.
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It could have been my imagination, but I DO think I felt a bit less friction between the string segments when using the flat.It was so barely detectible that it may have been my imagination, but I have to admit I think I felt it. Howver, there was definitely no snagging from string-on-string contact.
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I also did not tend to notice the string “grabbing” the response when using the flat. With the recessed response, I think the contact was less than I was putting forward. However, the only time I actually had a snarl-up in the gap (the response kicked in with multiple string layers) was while using the flat, so my theory might still have some merit, I just don’t think it’s as big as I was proposing.
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The profiled bearings performed very similarly. The “U” of the KK, the “V” of the Whipple, the “flattened to allow string movement” modified “U” of the CT… in a blind test, I dare any of you to tell which one is in your yoyo. 
I actually expected the Trifecta to perform the worst (worse than the flat, also), since in theory it almost “locks” the string into the middle (effectively cutting string mobility in half … in theory…) but it didn’t. The string must pop out of that groove with the slightest pressure (which makes sense when you think about it) and causes it to perform just as well as the rest of the profiled bearings.
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What WAS noticeable was the smoothness and sound of the bearings… they’re different from each other in that regard! The 10-ball was probably the smoothest, but the KK and Trifecta were good, too.
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Overall, although I do believe the yoyo “slowed down” more with the flat bearing. The tilting shows that there was more contact with the gap walls, something partially mitigated by the profiled bearings. But the slowing down was miniscule; not nearly to the level I was predicting.
Since the Skin the Gerbil trick didn’t produce very compelling results, I decided to first re-run the tests with a yoyo with a known narrow gap, the El Ranchero. Same results, really.
Since I was already in the testing zone, I changed up the trick, doing a quadruple-on Lindy Loop as straight as possible and then undoing it. They all seemed pretty similar again, including the flat. I suspected the flat was slowing down a BIT more, so I then did the quad Lindy Loop but just held it until the yoyo died.
The flat definitely died more quickly in THIS test, but again it wasn’t the yoyo’s response kicking in as I was predicting. It was the contact with the gap walls.
In conclusion: there’s no huge reason to prefer one over the other in terms of this string layering. When there was a slowdown with flats, it was because the yoyo was also tilting more than the profiled bearings. For this reason, I’ll still stick to my preference for profiled bearings… they help the yoyo maintain its plane better, which in somewhat artificial tests also BARELY prolonged spin time.
But that said, I also don’t think there’s a great reason to rush out to replace your flat bearings. If that’s what you have, just use it. It’ll be fine. And if the smoothness of a particular bearing (for example, the OD 10-ball) is more important to you than any perceived minor performance benefits, that’s still a GREAT reason to continue using it!
