It's an interface module for interrupts. I can't say if the "issue" is on the hardware, OS side, or more likely combination. My best guess is, that by disabling it, the OS will interface differently with the hardware. You may want to contact HPe directly if you want specifics--there may even be a fw update or bios tweak negating the disabling. That, plus the vendor should know best. In theory.
Did you check the Oracle Solaris Hardware Compatibility List (HCL), to see if it gave any other guidance for your hardware?
You might find more specifics to your answer here as to what the apix module is -- if the man page didn't help:
As well as info on APIC: