First of all, the Estes C5-3 is 7.79 N-s (see
https://www.nar.org/SandT/pdf/Estes/Estes_C5_2019.pdf) and the Estes C6 is 8.82 N-s (
https://www.nar.org/SandT/pdf/Estes/C6.pdf) You are correct that 9.8 N-s is the certification total impulse value for the C12FJ.
I've flown altimeters in just about everything. There is some magic in the Q-Jet C12s as they carry most any model, especially a heavier one, quite a bit higher than an Estes C6 or C5. I just went back in my logs as the 1/200 Saturn V is one I didn't fly an altimeter in every time, and I haven't actively flown for five years. So back in May of 2019 I have these data:
Flight 1 - (May 5, 2019) Estes C6-3 altitude 199 feet (via FS Mini)
Flight 2 - (May 12, 2019) Q-Jet C12-4 altitude 348 feet (via FS Mini)
Flight 8 - (July 11, 2019) - Estes C6-3 altitude 212 feet (via FireFly)
Flight 9 - (July 16, 2019) - Q-Jet C12-4 altitude 344 feet (via FireFly)
Flight 10 - (the same day) - Q-Jet D16-4 altitude 433 feet (via FireFly)
I haven't found any entries where I've flown this model on the C5-3 with an altimeter, though I thought I had. I may still keep looking but right now I want to go fly before the rains come back this evening.
So I exaggerated a little when I said "twice as high".
I've seen similar results with the Estes MAV, which is another heavy plastic model, though not as heavy as the little Saturn V.