I am having trouble finding a document, a guide or a whitepaper on this "IBM Customization". I am looking for information about what the image includes and what features/benefits I get by using it. At the very least I can assume it adds specific IBM hardware drivers, and I need to know which ones and what versions; does it also include features in addition to drivers that can be taken advantage of during operation: what are they, how to configure and use them?
Can someone point me to this info if it exists?
Well, I don't know about a document on that particular image, but those OEM images are just ESXi with specific drivers bundled.
The good news is you can inspect the IBM image yourself using the Image Builder CLI. You can even customize it to your liking, if you don't like what they've included/excluded.
One option would be to grab a vanilla image and the IBM image, and use Compare-EsxImageProfile to spit out the differences.
I am no expert on this but our company has a x3650 M4 and the extras I have seen is the support for the hardware, RAID card & sensors. From in vSphere you can see all the sensors and get the warnings/faults etc. You also get full control via IMM2 if you use that management interface.
I tried the ESXi 5.1u1 before IBM got around to publishing their own and it worked but any specific items for the hardware was unknown.