Engine limiting factors

agreed !

I have a ground-adjustable prop that never gets near 5800 at WOT even on takeoff (home field is 350ft), so I think it's probably a bit over-pitched.  However, I already have an aircraft that takes off in quite a short distance, and I do seem to be fairly fuel-efficient in cruise.  I had been considering repitching, but I don't really need more STOL capability, and I think my cruise burn will probably go up if I do it.  Is this correct thinking?

As Bill said, if you want to get 100 horsepower (and maximum thrust) out of the 912, you must also get 5800 rpm.

If...

It is to your best interest to be able to extract every last bit of power fron the engine that you can.
TakeOff, Clearing the  trees, Clearing a Ridge, and High Density Altitude.are a few common conditions that will have you wishing you had a few more horespower.

It is not uncommon for a typical 912 powered aircraft to weigh ~1000 lbs and climb at ~900 fpm.
This indicates that 70hp is need just for level flight and the ramainder produces the climb.
Giving up 10hp is giving up 1/3 of your flight power margin.
At 1000lbs, every One(1) Hp reults in a 33 fpm change in climb performance. This isn't trivial!

My recommendation is to pich the prop for 5800rpm at WOT in the initial climb at Vy with the understanding that you will need to reduce throttle to prevent overspeeding the engine as you level off or descend.  Something you probable intend to do anyway.
As you gain altitude you will eventually return to the point where WOT does not produce 5500rpm and the engine will again be making "All That It Can Be!"
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Fuel consumption is proportional to POWER.
Power is RPM time Torque(MAP).
Operating at a higher RPM at a lower Torque will produce the the same power and same fuel burn.
The lower Torque will produce less stress on the engine and it will run cooler and last longer.  A Plus-Plus Situation!!!

If there is a logical argument to be made for not being able to produce 5800 rpm at some point in the flight envelope, I would love to hear it.
If you intend to fly at Reno, I can understand where you might want to spend your 5 minutes at 5800 rpm in more or less level flight, but at least you made 5800 at some point.
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IF... You do not want to use all the power available, that is one thing.
IF... You do not want the power to be available at all,  That is like going into work and asking for them to reduce your pay because you do not want or need the extra money.

Excess availabe Power is like money in the bank.  The day wll come when you will need every penny!