Hmmm. Maybe I'm missing something... or maybe I'm just stubborn.
However, I think all I can give you is a qualified maybe. I will admit that the two subs are much closer than I had recalled. However, the evidence is not conclusive for Mirage as you seem to suggest.
The response curve for the Energy S12.3 you provided - and I believe that the Mirage S12 is the same sub in a different set of clothes - indicates that its "in room" usable output at 15 Hz is 90 dB. According to an October 2004 review that Ed Mullen did for Secrets of Home Theater and High Fidelity, the SVS PB-10 has an "in room" output of 94 dB. That would seem to indicate that the BP-10 does go a bit lower... though obviously they were not tested in the same room.
Published specifications for the two subs list the "anechoic" frequency response for the Mirage S12 as being 19 Hz-120 Hz +/- 3dB; and for the SVS PB-10 as 18 Hz - 100 Hz ± 3 dB and was measured as being a nearly flat ±1 dB from 19 Hz - 150 Hz. This seemes (to me) to indicate a potential for a slight win for the PB-10... though certainly not a runaway game.
Both reviews do indicate that the respective sub has usable output down to 15 Hz. The Secrets of HT&HF review indicates that the PB-10 "measured an excellent ±5 dB from 15 Hz - 100 Hz"; while the Audio Ideas Guide review states that the Energy S12.3 "is down only a couple of dB at 15 Hz, relative to 100 Hz". The graph appears to indicate that it may be down 5 or 6 dB from 20 Hz. That sounds pretty close to a dead heat.
Contrary to my previously stated opinion, I would say that the in-room graphs also indicate that the SVS PB-10 may actually be the max SPL winner between these two. Though we may be missing a set of matching graphs to know that for sure.
Anyway, I will concede that the Energy S12.3/Mirage S12 is a better sub than I had remembered it being, and I would conclude that I don't think you could go too far wrong with either model. Is that concession enough to keep the peace?