This is where it gets tricky, since there's still a lot of things higher on the list than SWD: Refreshing dots, casting Mind Blast if you have an orb when Empowered Shadows is about to expire, and then the big question is, if you DON'T have any orbs and Empowered Shadow is about to drop off, is Mind Flay still a better choice since you need to get an Orb?
Plus Mind Flay also reduces your shadowfiend cooldown. When you have no orbs and dots are refreshed, what else can you do but spam mind flay? You could mind blast to keep up replenishment, but I doubt that's worth it. Spamming Mind Flay will give you an orb fast enough.
I do think it's a legitimate question of whether you want to let dots expire and just spam mind flay until you get an orb, or refresh your low quality dots without empowered shadow. The answer certainly depends on how much mastery you have. The more you have, the larger the DoT damage differential from having the buff, and the sooner you want the buff up.
You'd think that extra haste would also encourage orb fishing because you have more orb proc chances per second from mind flay (since it casts faster). But you're also losing proportionate DoT damage from the fact that your hasted DoTs aren't on the target.
Anecdotally, when orb fishing I've seen that you normally get the orb pretty quickly but there are some long tail cases where you're fishing for a very long time without DoTs up (except obviously shadow word pain). This suggests that orb fishing has a high chance of causing a minor DPS increase but a low chance of causing a massive DPS loss. The optimal cast pattern could depend on your tolerance for damage variance in a particular section of the fight.
For example, suppose you can choose one of these two damage profiles:
Orb fishing: 90% chance of 16k DPS, 10% chance of 10k DPS (15.4k avg)
Refresh DoTs: 100% chance of 15k DPS
Then suppose you have a burn phase where you need to break a damage shield to interrupt a cast, and you need to sustain 14k DPS to do so. Choosing the Orb fishing option is ludicrous because you have a 10% chance of wiping due to low damage. But in a fight without a short term DPS threshold, the outliers should be mitigated by a sufficiently large sample size to get an overall DPS increase.
Again, that's assuming Orb fishing is even a DPS gain, which we'll have to simulate to determine.
On another note, I have a sneaking suspicion that because there are so many internal tensions in the rotation, the question of how you prioritize the lower priority options doesn't matter much. The high priority things are obviously "keep up your dots", "keep up empowered shadow", and "shadow word: death in execute range". It might be that "Mind Blast when you have 3 orbs" isn't a huge deal, given the opportunity cost of a Mind Flay and all its associated benefits. It's not like blast hits THAT hard.