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Here's a suggestion for SM16 with the new UI...
The Sender Priority Overrides is found under Settings->Routings. That doesn't seem to fit as it's a user-specific feature, but it's mixed in with Incoming/Outgoing Gateways, etc.
Drop this SPO feature (does anyone actually use it?). We've tried it and it seems like email was delivered at the same time as other messages. 99.9% of the time the spool should be caught up. Maybe it was created back when we had 128K connections and some mail had to be delivered before others? Seems like just another unused feature with more code to maintain.
Move it to somewhere you'd expect to find it. Since it's a user-specific setting, move it to Domain Settings->Users->Edit and put it on the Throttling tab with the other user attributes that define how messages from that user are sent. On this screen, add Message Delivery Priority fields -- a number drop-down and a description.
Bonus with 2nd option: when that user gets deleted, the settings gets removed. With the current method, you could end up with users that don't even exist in the SPO list.