I am rebuilding a 911 engine for which metal parts went into the oil pump. The impellers and case show some scoring and I will probably be replaced. Still it begs the question: how bad can it be? The scoring between impellers can be honed flat to prevent more galling. The scores between the impeller edges and the case obviously will allow oil to leak past. To that I ask - so what? I can readily imagine a reduction in output and therefore oil pressure. On the other hand it is hard to imagine a really dramatic reduction in pressure due to oil leaking past/through some scratches.

Do scratches tend to propagate wear in the pump?

When oil pumps fail, how do they fail.

Thanks folks, and note I am not asking whether or not to replace a pump, I am asking what happens when you don't. Or how bad does it have to be before it matters.

https://www.glennyeemotorsports.com/gt1_oilpumps.html

Which may be the wAy I go, but doesn’t answer my question. Thanks though.

Did you "run" a cam due to a blocked cam oiler tube ?.
Damage to the gears and pump body will result in reduced function of the pump on both pressure and return (scavenge) sides,the return side is less important but once the oil heats up you may well find the pressure side is delivering less oil pressure @ hot idle.
Also there will be a delayed time before the system primes due to oil leaking past the gears which IMO is caused by wear/damage to the pump body and not the gears.
Good luck.

Feel free to post a pic and you'll get some opinions on the wear. In general, if you see scoring or pitting on the gears or the housing, the oil pump is a paperweight. As mentioned, it likely won't make proper oil pressure. I've heard that Glenn Yee can rebuild and possibly exchange some parts. Depending on the condition of yours, that may be an option, or just find a good used one. Good, used early 911 pumps are not terribly expensive.

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Did you "run" a cam due to a blocked cam oiler tube ?.
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Well, no. I was really stupid. The nut came off of #1 intake rocker and jammed between the case and a cam lobe. The timing pin sheared through the cog teeth as the cam came to a halt. #2 & 3 pistons have valve imprints. That nut is going to cost me about $5000. Dumb dumb dumb. Because of course I was the one who didn't tighten it enough. It was on a hot day in Montana, and I didn't respond to the change in engine sound properly.

New pumps are over $1,000, so I am trying to avoid that.

Good used early 911 oil pumps can be had for $100-200

Supertec performance can rebuild your pump