I've done this mod and the following:-
XH snorkle - IMO don't bother unless you are just after the looks.
The minimum cross sectional area of the XH snorkle is the same as the original ED snorkle. The volume of the ED snorkle is actually greater. In theory, the flow through the ED snorkle would be better than the XH one. There was no gain I could detect (it was $26 not well spent).
Anyway, to fit it you will need to cut off the mounting sections (where the ED snorkle is mounted) off the radiator shroud. It will just sit in place, no need to secure it down. I have EF thermos fitted in my ED so no issue for me. (PS next mod I am thinking on is an EL intake manifold into my ED - better low end torque and slight top end improvement).
K&N filter - definitely an improvement (per my ED XR6, previous AU XR6 VCT and current XR8). They do deliver better airflow.
Note that you will need to clean the filter regularly, say, every 2 months dependant on conditions. No more noisy than a standard filter. You will feel an improvement in suction if you choose to swap filters back and forth enough times.
Airflow improvement - best improvement I have found has been from fitting the EA/EB MPFI flex tube. This is the one than goes from the throttle body to the bottom of the aircleaner box. The original ED one has a venturi integral in the airbox end of the hose and is about 40% smaller at the airbox end. The EA/EB one is the same diameter as the thottle body at both ends.
Fitting the earlier one delivers a definite improvement, one can feel the extra suction, is a tad more noisy but not by much.
They are fairly readily available from the wreckers (I paid $10).
The XR family ---->
Muzza: a XR8 AU seriesIII...bog standard :)
TBH: (the better half) drives an ED XR6 wagon (Mock Dev 4 160rwkw, JMM extractors, 4 electric windows, Ghia door panels in XR trim, rear head & arm rests in XR trim, Momo s/wheel with Cruise, EL thermos, EBGT wheels, etc) things to come soon = EECV upgrade.