According to that logic, a lower water level in the bowl should produce a better flush, since less water would go toward refilling the bowl.
I think you read my reply wrong since your argument doesn't apply to what I said.
Think of it in this extreme case.
- bowl of toilet is completely dry. Now flush.
Is this a more powerful flush than if the bowl was filled to the correct level? Is it as powerful?
The answer is no - the water dumping from the tank will fill the bowl and THEN push the contents out. If it takes 1 gallon to bring the bowl to the correct level, in the extreme case presented you're now only using 0.28 gallons to flush the contents for a 1.28 gpf toilet.
So, not having the correct water level in the bowl CAN affect the flush.
In regards to the original poster - if it's the bowl, couple of things come to mind.
- venting (I have no experience here, others will need to chime in)
- are the guts of the toilets the same? (did the upstairs toilets get new fill valves?)
- is the fill hose dumping into the overflow tube as it should be for the upstairs toilets?