Well, I finally found time to do several testflights:
It seems quite normal that the consumption is doubled, if you fly with Mach 1.5 (or slightly above) compared to Mach 1.1 with RPM = 95% (not the indication in the cockpit). With the Flanker I had the same issue, which is a PFM model. But even at Mach 1.3 the fuel consumption is about 53% higher compared to Mach 1.1!
With RPM = 85% your consumption raises from Mach 1.1 to 1.3 and 1.5 not that much, BUT it´s not the most efficient power setting to achieve max range with supercruise!
Wiki says that maximum range with supersonic speed without afterburners is about 1500 Km (in russian Wiki up to 2000 Km and with 11100 Kg internal fuel, but that´s with the older engines). So with this extra fuel I got a maximum range of 2080 Km with RPM= 95%, on an "optimal" altitude of 15 Km, IAS= 760 Km/h, TAS= 1980 Km/h, Mach= 1,1, flight duration: 63 minutes.
So the power setting of 95 % and Mach-number 1.1 are most efficient when it comes to range with supercruise. And I compared the RPM-settings of 85%, 87%, 90%, 95% and 100%. (I decided to put Mach 1.1 as the minimum supersonic speed).
I also modified the fuel equation in the lua-file, but the actual consumtion in flight never changed, neither with speed, nor with altitude (on Su-57 and Su-27 as well).