I have heard it is possible to leave it in but u need to undo it as it is bolted to the gearbox
Thinking on - to remove the IRD, you also have to remove the 2 cooling hoses which some clever people have managed to put bungs in - but me, I got wet and needed to refill quite a bit of coolant!
All in all, if the IRD can stay in place (maybe by supporting it) it will be a quicker job - but I don't know if the gearbox can actually come out (due to splines - eg space and allignment when reinstalling) with it there.