Thanks, your chat with Intel support had all the clues, but it took me 20+ tries to pick up the key points I ended up with this more elaborate script.
Highlighted the key points.
Issue: ESXi 5.5 upd 2 does not install on S2600CP4 with 'EFI optimized boot', hangs in boot on Initializing ACPI.
Use build VMware-VMvisor-Installer-201501001-2403361.x86_64.iso
Target HDD on onboard SATA-2 (white connector), all adapters pulled
BIOS: F9 reset
Advanced > Processor configuration> leave Processor C3 disabled, enable Processor VT-x options
Advanced > PCI configuration > Memory Mapped IO above 4GB: disabled
NIC configuration > disable Port2, Port4, Port3 PXE
Boot options > EFI optimized boot: enabled
Boot ESXi ISO via UEFI CD-ROM
Install as usual, update to build 2456374 via patch ESXi550-201504001.zip
re-install adapters