Chrysalis,
"...The question is tho as the bios assigns irqs on a apm/acpi basis ie. only up to irq 15, is the irq still been shared when windows APIC kicks in and irqs above 15 are been used?..."

yes and no, and to make sense of why this is, some links below:
- as an example, from KB314086:
"...The Plug and Play operating system settings in the computer BIOS do not generally affect how Windows XP handles the hardware. However, Microsoft recommends that you set the Plug and Play operating system setting to No or Disabled in the computer BIOS. For information about viewing or modifying the computer BIOS settings, see the computer documentation, or contact the computer manufacturer..."

A general description of IRQ sharing in Windows XP
http://support.microsoft.com/kb/314068

How to force a Hardware Abstraction Layer during an upgrade or an installation of Windows XP
http://support.microsoft.com/kb/299340/

HAL options after Windows XP or Windows Server 2003 Setup
http://support.microsoft.com/kb/309283/