Is there a benefit to holding a gym longer than 8 hours, 20 minutes?

None whatsoever. Neither holding more that one gym. The ONLY benefit is that you might be upgrading your badge and getting a few dozens of stardust.

(This answer is best read in the voice of Doctor Evil, particularly when you see Quotation Marks " ".)

The "Benefits" to holding a gym for more than 8 hours include (and are entirely limited to) the following:

Number 1: After you "hold" the gym for several hundred hours - which, as you may have noticed, is slightly easier after the "improvements" - you will be "awarded" with a Gold Badge for that gym, and the Gold Badge allows you to get extra items when you spin that gym's Pokestop.

2) The "Benefit" to feeding Berries to friendly Pokémon is that you gain XP on that gym (refer back to Number 1 to see how that XP "benefits" you) and that you gain 20 Stardust. So if you feed 10 berries to Pokémon on Gyms, you will have enough Stardust to raise a Level 1 Pokémon to Level 2. "Yay". You may also receive a Berry that corresponds to the type of Pokémon you fed - so if you fed a Blissey a Berry, you might get a Chancey candy. Uh huh. Riiiight.

Number 3: This is the only real benefit for feeding a candy to a Friendly Pokémon, and that is to be Evil. MWWWHAHAHAHAHAHAHAA! Oh, we fed berries to the Pokémon on our Gyms, and then we laughed and we laughed and we laughed because every time we fed a berry to a Pokémon, that Trainer was farther away from collecting his coins! How delightfully Evil - it could only have been dreamed up by my son Scott.

Well, dreamed up might be a bit of an overstatement. He was probably being rebellious and trying to "stick it" to Daddy and be "Good", but you can't deny what you are - and he is Evil. It's right there in his name, Scott Evil. See? Scott. The most Evil name I could think of. Mwhahahahhaaa! I wonder if he's working for Niantic right now, or if he's just a Freelancer? Hmm...

(cough Sorry, got a bit carried away channeling Niantic there. I mean Dr. Evil, sorry.)

Niantic's brilliant scheme to get a higher user count - i.e. more alt-accounts to clear your rotting PKM from the gym.... I hope I'm wrong and they fix this perverse incentive.

I think there is an obvious answer that people seem to miss.

If you hold gyms for extended periods like several days, then hopefully they will be returned to you in a spaced out manner, so you're collecting 50 coins/day.

So if you hold 10 gyms right now, and one of them is returned after 9 hours, then you hope to hold those other ones for days so one a day is returned to you.

So to me, my strategy is to be in a bunch of gyms (5-10...but not much point in more than 10), but NOT put a lot of time and effort into it.

There could actually be a benefit. Have you already collected your 50 coins for the day and will feeding her meaningfully improve your odds of her lasting until tomorrow (or some future date)?