Well, first off, my idea to get a sword that gave a TP return of 5 didn't work out. Turns out, I'd need 180 delay to get a TP return of 5, and there's no sword in the game with a delay that low. <_> So, all of these tests will be slightly over 100 TP, I'll list it for every test.
Note: All of these were solo, so consider the initial pulling enmity.
Test I)
1 Dispels
30 second wait
0 Enmity
Atonement at 102% TP: 46 damage
Test II)
10 Dispels
30 second wait
0 Enmity
Atonement at 103% TP: 306 damage
Test III)
20 Dispels
30 second wait
0 Enmity
Atonement at 103% TP: 360 damage
***The result greatly surprised me. Perhaps there's a limit on damage from purely CE? I retested it and got the same result: 360 damage.
Test IV)
32 Dispels
30 second wait
0 Enmity
Atonement at 103% TP: 360 damage
***Clearly a cap.
The next set of tests were somewhat difficult because it required building 100% TP and then losing aggro on a mob without zoning. I tried it in Cape Terrigan and it was pretty much impossible since there's aggro everywhere. I trained around my bunny for like ten minutes but because I couldn't get much distance on him due to constantly zig-zagging past Goblins and Raptors I couldn't get him to despawn (I'm sure this was a pretty hilarious sight if anyone saw me).
Instead, I opted to go somewhere where I wouldn't aggro anything, yet was still targetting mobs strong enough to deal damage to me: Western Altepa Desert (Target: Desert Manticores)
Test V)
1 Silence
Waited for ~400 damage
Atonement with 122% TP (due to getting hit): 0 damage
Test VI)
1 Silence
Waited for ~500 damage (TPed up to like 60 on a seperate mob and let the target manticore hit me until 100% exactly)
Less than 1 second delay after using Sentinel
Atonement with 100% TP: 134 damage
Test VII)
1 Silence
Waited for ~500 damage
1-2 second delay after Sentinel+Invincible
Atonement with 100% TP: 360 damage
***Again, seems to be a cap! Which is odd, because 360+360 is only 720 and I've done tons of 750 Atonements. <_> Anyway, these are the results!