Every action has a reaction, but I think the arguments on both sides of this one are over reactions. I rode Derby > Barcelona some years ago, set off on a pair of 30mm Marathon Racers and split the rear before leaving the UK, choice in the first Decathlon in France was 25mm or 40, nothing in between and I bought the larger. That wouldn't fit the frame so the bigger tyre went on the front. That's a difference in radius of 10mm. Did it feel different? Yes. Was it a problem? No. 50 miles later I'd forgotten about it, not only did I not go looking for another tyre in the remaining 900 miles, I didn't bother swapping it when I go home.
Gratuitous photo, spot the difference
(https://live.staticflickr.com/65535/53746029719_83a7ce4829_c.jpg) (https://flic.kr/p/2pTms9P)
A 27.5" rim has a 25mm larger diameter than a 26" (584 V's 559), 3.5%, a radius difference of 12.5mm. Form your own opinion about the effect that might have, or better still try it yourself.
Opinions on bar bags also vary, though you have to factor in familiarity. I nearly always ride with one, out of laziness it'll sometimes hold a 2kg chain lock as well as the other stuff, the only time I consider it an issue is when off the bike it makes the bars flop. The weight difference with/without the bag will be less than the difference in weight distribution between grips and bar ends on a straight bar, or tops and hoods on drop bars. Your choice of course, though it makes me laugh that many Audax riders who wouldn't have considered one a decade ago, now wouldn't be without.