It looks like disabling ACPI also screws up the plug and play or device discovery, I would think that would cause more problems.
In most cases, ACPI prevents Mach3 from running properly, and changing to Standard will usually fix this. Both PC's I have will not run Mach3 in ACPI mode, but work fine in Standard mode.
Well the 400 seems like a lot of extra wiring and work for a benefit I do not need.
The UC400ETH require no more wiring than a UC100. It just requires a ribbon cable to the G540 instead of a parallel cable. About $5. And an ethernet cable instead of USB. $2.
Ethernet provides a more reliable connection, and is more immune to noise issues.