Well, I'm sure this one's complex too, but I'll give a shot with my thoughts.
I'm not positive on how the damage degradation works, but I'm assuming it takes the flat percentage off the top value always. So 360-x%, 360-xx%, so on so forth. As opposed to 360-x%, 324-xx%, so on so forth. So Tenka Goken, under that assumption, is 1440 without Kaiten (5 enemies), and 2160 with Kaiten (5 enemies). In this scenario, Kaiten has given you an additional 720 potency. Kyuten has no degradation, so on 5 enemies it's worth 750 potency. So as far as Kenki goes, since it costs 5 more to do Kyuten, it seems just about as efficient as far as resource spending goes.
You can use Kaiten or Kyuten, but Kyuten+Raw Tenka will do more overall damage (by 30). And even though Kyuten costs more Kenki, it's not until 4 uses that it'll have cost you an extra Kaiten. That is, 4 Uses of Kyuten requires 100 Kenki, 100 Kenki could be used for 5 Kaiten. We need to get to 100 Kenki before worrying about that, though. Our AoE combo gains us 15 per Fuga+Followup. We'd need to do that (basically) 7 times to get 4 Kyutens, or conversely, 5 Kaiten buffs.
So. By 7 reps of the AoE rotation we WILL get 4 Kyuten's (we've generated 105 Kenki). We will NOT however, have the chance to spend 5 Kaitens, in this same time frame. Because 7 reps, while gaining us the Kenki to perform that, will only have gotten us enough Sen for 3 Tenka-Gokens, max (and therefore, only 3 opportunities to use Kaiten).
Altogether... We have a few options here. 4 Kyuten+3 raw Tenka. 3 Kaiten-Tenka+1 Kyuten, 2 Kaiten-Tenka+2 Kyuten.
Option 1 is (750*4=3000)+(1440*3= 4320)~ 7320 potency.
Option 2 is (2160*3=6480)+(750)~ 7230 potency.
Option 3 is (2160*2=4320)+(750*2=1500)~ 5820 potency.
So, again, napkin math, but it would seem that Kyuten spam is stronger in situations where you have 5 or more enemies. 4 enemies or less, Kaiten gives you a better benefit (by a small margin), if I understand the degradation on Tenka-Goken correctly.