Going with the upper end:
5,475*60*60*24*299,792,458= 1.4181382e+17 meters (about 141.81 quadrillion meters)
1.4181382e+17/ 9.460528405e+15= 14.9897326016
Seeing that the Normandy is capable of flying a distance of roughly 15 lightyears in a day rather is rather impressive.
If, however other ships are capable of the same thing, doesn't that make the Normandy feel kind of underpowered. I mean if you are supposed to be the fasted ship you might as well do it right.
Yep, like I said, 5,475c - roughly.
And the Normandy is probably exceptional. Case in point - Ashley does list 12 light years per day to be an average speed for Alliance vessels at least, and the codex lists 30 light years per day as the Reaper speed, and that this is twice as fast as the fastest vessels.
One can infer from that, that the Normandy is ONE of the fastest vessels in the galaxy - but that there are others that can travel that fast as well. That does not mean that every ship can travel that fast - indeed, one can deduce from in game dialogue that most ships do not travel that fast at all.
So, not only was the Normandy one of the fastest ships in the galaxy, but the stealth tech gave it an even further edge. So yeah, it was unique. At least for a little while until a second SR1 class ship was built, then the SR2, then the Salarian stealth dreadnoughts and the Quarian stealth vessels.