_without_ IRQF_ENABLED: where an IRQ handler can take a long 
time to execute, the handler can be converted to a threaded IRQ 
handler - where it's fine to enable IRQs as there are no stack 
nesting issues.

Only if you can mask the interrupt on the APIC without losing it or
having the APIC throw a fit.
 

So there's no real technical problem here.

In the long term no - but forcing people to make sudden changes to
critical I/O drivers isn't the right way to do it.