Lpc2000

I'm trying to work out some interrupt response times for the LPC2000 family. 
My main concern is with the response to timer interrupts.

I can work out exactly what the range of response times should be in 
"Normal" (full power) mode.

What about Idle mode?

There won't (obviously) be an instruction executing, so I don't need to wait 
for this to finish.

Is there any other source of delay?

I don't know enough about the underlying hardware to know what the 
difference is - at a silicon level - between Idle mode and normal mode, and 
whether it takes time to make this switch.

Can anyone help?

Thanks,

Michael.

PS.
I can, off course, take some measurements (and intend to do so).  But I'd 
like to have a better understanding of the theory too.