"...The top-end EC12 has z12 engines that are 25 per cent more powerful, at around 1,600 MIPS..."
With all those IBM engineers struggling to build a new high performing Mainframe cpu, it is a heavy burden to try to keep up with the old back compatibility from 1960 or so. That is maybe the reason the new Z12 Mainframe cpu gives only 1,600MIPS? And the previous IBM Mainframe z196 cpu from 2011, was dubbed the fastest cpu in the world, by IBM. Thus, the Z12 which is faster. should cement the IBM claim "worlds fastest cpu" even more. Probably, IBM will soon claim the z12 is the fastest CPU in the world.
An old Intel Nehalem-EX cpu gives 400 Mainframe MIPS - under software emulation using "TurboHercules". Software emulation is 5-10x slower than running native code. If Nehalem-EX could run IBM Mainframe software natively, it would give 2000-4000MIPS. This is faster than the "worlds fastest cpu" Z12. Here are sources:
Thus, if you had 20 of the old Nehalem-EX, they would give 40,000-80,000 MIPS, matching the biggest z12 Mainframe. For a fraction of the price. Of course, the IBM Mainframe have better uptime and better I/O - no doubt. But cpu performance lags behind, even though they are "the worlds fastest cpu".