I suspect the reason for the change was not due to reliability, but due to the N8008 shutter blades being cheaper to make. I've had the fortune (er, misfortune?) of owning a couple FM2 series bodies that had damaged shutters. One was an early FM2 with titantium blades that had one blade that kept coming loose when the shutter was cocked. It would fire normally, and was light sealed when not cocked. The other was a late FM2n with aluminum blades. Had definitely had the finger poked through them, and were crudely bent back into shape by whoever put their errant finger through the shutter. The shutter worked, but even if you had the mirror down and a lens attached, you could still see some light leakage through the shutter. That body had other issues that caused its demise, namely a meter that only displayed (+), no matter what setting, due to the ISO dial being fouled up - surprisingly, a repairman was able to fix that and transfer the shutter speed dial assembly to a well-brassed FM2n that I owned at the time, which had similar meter issues, due to a broken resistor element.
Anyway, what I remember of the early titanium shutters, was that there was a bushing, used in the original FM2 and very early FE2's (and FA's) that would wear out, if there was repeated use of 1/500 and another shutter speed, which would cause the shutter to fire at 1/1000 when set @ 1/500, and I think 1/8000 @ 1/4000. Would cause the shutter to show the "finger" dents in the blades without someone actually sticking their finger through said shutter. By the time the FM2n came out, Nikon had redesigned the blades and the bushing, to prevent that problem.
With respect to the top speed being so far off, I'll have my local tech check out my FM2n when I pick it up, and see how far off it is @ 1/4000. It's possible that the shutter speed tester that tech in Minneapolis had didn't handle 1/4000 properly. May've been designed for shutters, like the F2/F3 shutters, where the top speed is 1/2000.
-J
-J