I can't remember ever using or relying on this. The only purpose of Osiris's Temple Complex for me has been to use the Altar of Sobek. Even the Oracle of Min has a limited real impact on my designs.

I'd wonder what the practical effects of this are? Playing without prior knowledge, you can't know what the map's flood quality is set to. Maybe after 4-5 years you can get an average indication but I'd say that's too late to be helpful.

Most important to know is probably the ceteris paribus average yield over a floodplain with different flood quality settings. That would be very dependant on efficiency of the setup, and layout of the floodplain, and your talking about averages over several years rather because quality varies year to year. Only then can you tell what impact the TC has on any given map.

I'd probably say 2 things need to be checked out - what's the average yield on an average flood plain, and what's the average yield on an Osiris TC enhanced flood plain? But that should just apply to any Sandbox map with different flood quality settings. I dont think it's very useful to know because actual yield is so map and layout dependant, a 1 better change in quality shouldn't really matter, should it?

the Oracle of Min has a limited real impact on my designs

On some maps, Min's Oracle has a large impact, since it doubles the yield of hunting and fishing.

I'd wonder what the practical effects of this are?

If there is a wide floodplain, where a "poor" flood should cover river-touching farms, Osiris's temple complex means that irrigated river-touching farms should always have 99% fertility. Without Osiris's temple complex, a long series of "none" floods should reduce the fertility of all irrigated farms, maybe all the way to 22%.

The opening post is not entirely correct. Building Osiris's temple complex (OTC) does improve the Nilometer by a level but that only increases the flood about half as much. For example, in the Sandbox custom mission, if the Nilometer says no flood without OTC, building OTC raises the Nilometer to poor flood, but the flood only covers 3 tiles from the Nile, compared to 6 tiles that are covered with a poor flood without OTC.

Flood quantity depends on the Nilometer, the maximum distance of a floodplain tile from the Nile, and whether OTC exists. A flood covers those tiles that are no more than a specified fraction of (the maximum floodplain distance plus 1) from the Nile. Without OTC, the fractions are: none=0, poor=.2, mediocre=.4, good=.6, excellent=.8, and perfect=1. If OTC is built, a perfect flood is not changed but for the other levels, the Nilometer increases by a level and the fraction is increased by .1. With OTC, a perfect flood on the Nilometer might not completely cover the floodplain.

I tested all flood levels in five missions (with the maximum floodplain distance): Heh (4), Sauty (20), and the custom missions Henen-nesw (11), NAFTA (15), and Sandbox (29). (In Heh, OTC did not affect the flood at all.)