I know this is not the answer you are hoping for, but... The Precision T5400 is the final Precision Tower that used Core2 based Xeon processors and DDR2 RAM.For your towers you would need to stay inside of the Xeon 5400 family with the fastest being the X5492 (3.4GHz quad core) To use the Xeon X5550 you would need to upgrade to the Precision T5500 or T7500.  
Hello from Germany!Im running 3 Precison T5400, BIOS A11 (latest) with Xeon E5420 CPUs, two CPUs in each machine.For VMWare ESXi 6.5 (or newer) I need newer CPUs - like the X5550.Will the T5400 support the X5550 CPUs?Best regardsPeter
T5400 CPU Upgrade to XEON X5550 possible/supported?