This may be obvious to most, but wouldn't fuel flow rates vary with altitude?
If so, would that be a downside (or additional challenge) to using fuel flow values for throttle settings?
Are there different fuel flow vs. altitude implications for the ULS vs. IS?
I am not an expert but I've spent a lot of time studying this issue, mostly for Lyco engines.
It's pretty well known that the IS version of the engine has better BSFC but that's really all that I know about it. When you introduce full FADEC including spark timing and mixture then it's not simple anymore.
The fuel flow relationship to power should be insensitive to air density (altitude, etc.) BUT it should be sensitive to the torque curve. In addition, prop efficiency varies with RPM and TAS but not as much as you'd think. That will be more with CS than FP, oddly.
The efficiency of the prop and of the engine (with or without boost) will vary with Density Altitude. But how much it varies depends on many other things.
If the air is thinner then, at a given air-fuel mixture, there cannot be as much fuel used and, of course, the maximum available power is thus less. But the fuel flow at a constant mixture will still be the big determinant of power. We make power by burning fuel.
The air-fuel mixture primarily, and other factors less so, will have a close relationship with engine power. The power as it affects performance is engine power modified (reduced) for prop efficiency. Some engines have quite flat torque curves in the regions at which we cruise. Some less so.
To the best of my limited knowledge, the 912 ULS has a reasonably flat torque curve and a reasonably flat mixture as controlled by the carbs. That is why fuel flow reflects actual power with reasonable accuracy. I define reasonable as useful for normal flying such as XC.
All gasoline engines have BSFC (Brake Specific Fuel Consumption) which we usually see expressed in pounds per HP per hour. My 7A with manual leaning, etc. could run at about .40-.42. Lower is better. The 912 ULS runs at around .44-.45. You can compute this from the data in the Rotax specs. I'm writing this from memory. BSFC is a measurement; you can't directly control it. It's the result of many things, even, especially, compression ratio. Not that you care, but my 1975 C-150 had a BSFC of about .44-.45 over a very large range of RPM's and Density Altitudes. I calculated that from the POH.
If you think briefly about the above paragraph you will see that a given BSFC means that fuel consumption "equals" power. I was able to see computed BSFC on my GRT in my 7A but it would be useless without the ability to control mixture. My Lyco IO360 had a BSFC curve in the "book" and it showed me what I was able to observe experimentally - that my best engine efficiency was at 8GPH (lowest BSFC). Unsurprisingly, the best cruise performance was at 8GPH and 8000'.
I'm sorry if this is TMI, but I tried to take your question seriously. It's fun to study this stuff, but simple read-outs like fuel flow are very good for low performance, fun airplanes.