Not sure if anyone is interested but here's a preliminary report....
Switched from the 1hp impeller to the 3/4hp impeller. Only ended up changing one seal. I was very careful with one and that one was reused. The other was corroded pretty good, so that was the one I changed. No leaks so far.
There is no noticeable flow rate difference. If I put my hand in front of a return after cleaning the filter, I can't tell the difference from last year.
However, when vacuuming with the larger impeller, I would get air in the skimmer basket and air in the pump basket, and I'm pretty sure it was air coming out of solution from the pressure drop. When vacuuming with the smaller impeller, I get no air in the pump and the suction is not noticeably lower.
The pumps are much, much quieter with the new impellers. My wife didn't realize I had turned them on when I did, whereas before, you could easily hear them from the house with the windows open.
Finally, we live in a very dusty area due to a horse stable about a half mile upwind from the house, and I was struggling with the "is it algae or is it dirt" problem. Both last year and this year I opened to a dirty non-swamp, and both years they cleared up in about a week of 8hrs pump time per day. The water seems to be as clear as last year, but this year the sediment settling to the floor seems to be less. I've had the solar cover on all week (it's 80 in April, but 65 in June, love Chicago weather), so I can't make a definitive judgement, but I can update in a week or two once the temp warms up and we really start swimming.
I haven't seen a power bill yet, but I think that even after spending $50 for two impellers plus shipping, if there's even a little bit of power savings, the improvement in the performance of the pumps and filters makes the project worth it. This is only my personal/anecdotal experience, so please don't take this as a TFP-wide endorsement, but if you have a situation where the pump is way over-sized for the filter (very common, it seems), downsizing the impeller would be a very simple, cost-effective way to improve efficiency without shelling out serious cash for a smaller pump or bigger filter.
@mas985, thanks for your help with this, your guidance helped me pull the trigger.