Other manufacturers are using heat treated alloys and have done for many years, Rover was the first when K series was introduced but virtually all have caught up and gone beyond what the K series achieved.
The K series is very light but that is largely down to minimal material in its construction however having seen the disaster that followed for Rover other manufacturers have played safe.
The head saver shim was not actually first introduced for Rover but Ford, the Zetec engine has very similar issues to K series and there are 2 functions to the shim, to even out clamp loads and reduce high stress points, even more appropriate on heads that have reduced in hardness and secondly to carry a heat activated lacquer that melts into surface voids that can lead to other under surface cavities. This is a a result of the high speed casting methods Rover developed and others now use. The head saver shim idea was shunted over to Land Rover (then owned by Ford) as a potential fix for the K series failures in Freelanders running the K18 engine.
Heads must still be within a hardness of 95BrHd to have any chance of being serviceable even with a head saver shim. If you calculate the load on the head in a K series during ignition stroke you will see that the thin stainless steel shim will not make much difference if the head is too soft.
In a true competition engine using these shims can cause issues as it will effectively lower compression ratios, in engines with no adjustment in cam timing it will also affect this, this is critcal to developing power. but perfectly good in a moderately tuned or road engine where long term service is important.
Still one of the most ignored but critical elements on the K engine is the liner protrusion heights. Solving the head issuse is pretty easy.
The success of the repair is that its not just 1 thing, it a range of elements that when all done correctly will give a relaible and high performing engine
Part of what you say is absolutely correct, hardness testing of the head to min 95 brinnell, and the cause of most failures. Many garages do not understand this nor did /do many tuners
I have spent many hours with the gasket engineers at Payen Federal Mogul and Rover talking to and learning about the gasket technology. The fire ring is very sophisticated and designed to work with an engine that experiences head shuffle - like F1 engines.
The shim came about in a round about way. Payen were obviously working with Rover who were supplying parts through WSR to Judd for the touring car. Judd had a particular problem with gaskets eating into the head at the rear of the engine. NOT the front, just the rear, and tried all sorts of things to address this, 3 dowels, elastomer to seal, etc etc etc. They could not stop the issue, until they made a saver shim. Thus Judd were the first to incorporate one on a K.
Unfortunately, as so often happens the cause of the problem was not only missed by Judd, but Rover and Payen were not in the loop because they were not actually working on the Judd engine, just trying to help on the phone. What had happened was that Judd had removed one of the lugs at the rear of the engine which normally mounts the gearbox in order to clear the Hewland diff. Now I new from Robert Allebon then responsible for cranks at Eastworks that extra webs had had to be cast in those mounting lugs for production blocks, because DL1 pre production blocks without that extra web were cracking heads, losing gaskets by firerings working into the rear of the engine because of the twist put on the rear of the engine by the gearbox, in a road going car with road tires. Imagine how much worse it was with 180lbft, a Hewland box and Slicks for Judd. When I saw that, the answer was to radically redesign the gearbox mounting with stiffening plates along the block that doweled to the bell housing. The problem went away.
However the shim idea was in the air. Ford who were after a "visible" solution to the "HGF" issue persued the shim idea through payen with MLS, the added befit that it would alow soft heads to be reused.
Ford did not understand the issues, and fought a political war to introduce MLS AND shim, against the advice and wishes of the Rover design team. Some I know were furious about that, and in fact MLS was never signed off, it never passed all the tests a gasket needed to to be signed off, but was just sold on by Ford into the dealers after the collapse.
Same is sort of true about the new fire ringless gasket. Spoke to some of the guys at SAIC, old Eastworks engineers who know about it, but have no idea who makes it, and I can tell you the chinese made K uses a fire ring identical to the one on the Payen gaskets. No one in the office yesterday, had any faith that the ringless gasket could ever work on any K.