Yes, applying the fix I suggested does limit the Guest to utilizing ONE cpu only, no matter what the VirtualBox settings are. Part of the reason it is a Partial Fix. For me, I am getting better overall WinXP Guest performance on VirtualBox 4.3.2 running with NO APIC and SINGLE CPU than with APIC and multiple CPUs.
Still looking for feedback as to why / what changed that caused the loss of performance. Hopefully we get a fix that allows us to go back to proper settings.