I see 600/800 thrown around usually.
Useful Action cycles:

Landings:

Max reward possible for P-51H: 1210/min. For 15 mins, that comes out at 18150.
At 1:55:19, I survived full 15 minutes with 1025 score and got 13222 SL, this is in ballpark of ~73% of the 18150 calculated. Then I landed and got 3.3K SL, which is ~18% of the 18150. Both are actually a bit below 80% and 20%. This also means that apparently ~9% of the reward is missing.
At 1:10, I survived full 15 minutes with 454 score and got 11477. This is 63%, landing represents 2869/18150 = 16%.
It seems a bit more complicated than a flat-out “score caps at 600”, but it seems to effectively cap there.
According to the original thread, Rewards for useful actions in the aviation SB mode - Simulator Battle Discussion - War Thunder - Official Forum
There is less of a “maximum score” and more the fact that it seems they use activity% for calculating useful action reward. Based on data in this post (Report on how scores, SL, and RP rewards are calculated), activity% is not a linear function and rather seems to have a maximum.
"ARB activity as function of score and time

On further research (Community Bug Reporting System), it appears that the 600 score consensus I’ve seen posted before is effectively a soft cap, 800 is also a soft cap. However, due to the non-linear way activity is calculated you will never exceed 92% activity no matter what score you earn.
ASB activity as a function of score against 15 minute survival

This also explains where the 9% of my reward went. Naturally, the bug reporting manager closed it as “not-a-bug”, so being unable to get 100% of your reward looks intended.
The max SL pay-out, it turns out, is even less than the theoretical maximum from the stat cards!
Some planes' reward% at 2k, 1k, 800 and 600 scores


This also explains why the belief that 600 is the max score has been something I’ve read stated and believed as truth. You receive 86% of the reward at 600 score, getting 800 gives you 90% and 1000 gives you 2%.
Quoting from the guy who did the researches:
200 points = 54% reward
400 points = 75% reward
600 points = 86% reward
800 points = 90% reward
1050+ points = 92% reward
Thus “2 kills and you effectively cap your reward” is true for practical purposes.