On the viability of S3's, I did not say they were no longer viable, in any case, there's the argument for S1 to S5 kits in the same suggestion (but of-course you conveniently skimmed over that).
I spoke with yoshi (by email) and they are considering an s1-s5 upgrade kit but not until they get some feedback on batch 1. Upgrade kits are not for inexperienced people and poor thermal paste application or nicking something with a screw driver could result in a few loud and disgruntled buyers who can't RMA a diy product that they damaged by improper installation.
My guess is upgrade kits will either have no warranty or be provided only in bulk packs of 5+ so that it isn't being sold to people incapable of correct use.
Lets be honest, the s5's are simply too little too late out of date technology. If underclocking them within the firmware was possible at 12v, they would have been a contender. Over the coming month with expected jumps of 20% or more, it's a loss leader. With no warranty, why would anyone consider this unless of course it was simply as a hobby, knowing you'd lose money anyway.