If you're willing to consider theoretics for 5g:
The best S2D result documented was with ambient memory and a Z97 Impact. Alex's 3k 9-12-12 was in the mid-low 5.56s and some potential to push further.
S1G strong enough to run comparable settings is already known to exist. Even with tRP being weaker, the stronger tRFC, tRDRD, and tWRRD should make ambient 5.55s viable. Sammy PCB likely wouldn't be used for this because of RTL and mild tRDWR weakness, however even B63 sticks should be free of this issue. Basically, golden ambient S1G should be in cold H2C territory.
On top of that Nox showed that tCL does scale with phase, and tRFC scales further. Combined with with a Z97 LN2 to make sure tRDWRs and RTLs stay low, I think even a good chiller or weak phase unit should be enough to push into the mid-low 5.55s. It should also be worth exploring if 3200ish 9-13-14 or similar higher frequency settings are worth running.
One more path would be running quad rank, which would probably be ideal on a T-top board like the 4-dimmers from Asus and Giga. I'm thinking with the same frequency and tRCD/tRP, the benefits from quad rank should about match the drop in time from the lower tCL and tRFC cold brings and lower RTLs a good 1dpc board provides. If someone could get phase + quad rank running right, 5.54s should be in range, but I would expect that to be quite difficult.
Either way, I doubt we're going to see 5.54s until someone starts PCB swapping in mass. A couple users here have demonstrated that it is possible without special tools, but even one IC is a pretty big time investment, let alone multiple dual rank sticks. Finding quality ICs isn't the problem, as multiple people here have large quantities of single rank sticks to bin through.
I think there is a future for S1G as a viable alternative to PSC/BDBG/BBSE, though it is still a ways away. As far as other ICs go, I think it's worth looking into S1E and maybe S1D, as well as revisiting 1GBit Micron and Hynix on Z97.