I meant that the Isp figures provided are based on the mass of the fuel alone, so the Isp will stay the same even if the case mass goes up. I was trying to explain why Isp alone doesn't tell you everything you need to know about the overall performance and efficiency, and I was doing a poor job of it.
Thinking about this some more, I think the best way to explain why the N5800 is "special" is just to use the
rocket equation to show what delta-V the motor could provide on its own. Delta velocity = Isp* G * ln (Mfinal/Minitial). G is just the gravitational constant, 9.8 m/sec^2 if you're working in SI units. You can plug in motor data from thrustcurve for any motor and this equation will tell you what the burnout velocity would be if your airframe didn't weigh anything or have any drag.
For comparison:
N5800 2095 m/sec Isp: 228
N1100 1814 m/sec Isp: 210
N4800 1557 m/sec Isp: 197
G80 1481 m/sec Isp: 223
I600 1408 m/sec Isp: 186
G115 859 m/sec Isp: 229
I'm curious now if there are any motors on thrustcurve that have a higher delta-V. I doubt it but I haven't searched comprehensively.