Sheilding mains supply for HiFi and HT

How effective is shielding the mains supply to my stereo and home theatre GPO, our sparky will be running a new dedicated line soon, 20Amp/4mm*2 and I am thinking of getting him to run the supply through steel conduit so I can ground the conduit to earth.

Lots of contention on this issue in HomeTheater shack and other forums and I'm looking for opinions, for and against etc:etc;
Run from the main fuse box is about 13M and a dedicated 20A CB/ELB will be used
 
You'll need some type of series filter to keep that garbage out of your system. While Corcom type RFI filters might help, I have found a shielded isolation transformer to be the ticket. I have an AC drive in my house to run the well pump; lots of transients visible on the line. Transformer knocked it out. An arc welder I believe has a lower frequency of strikes than a drive, but it should still work.

If you have an audible problem only when he first strikes his arc, then he is notching the line, and there is not much you can do about that. An AC regulator would be the next option, and not a very desirable one I'm sure.

Steel conduit helps to keep airborne RF from contaminating long runs of wire (antenna action), which is not your stated problem. Twisting the wires does almost the same thing, but that is frowned upon in building wiring.
 
A shielded twisted pair is slightly more resistant to receiving and transmitting radiation than a twisted pair.
A parallel pair is one stage worse than those.

Adding a shield to all your mains cables may give a measurable improvement in line noise by shielding radiated noise.

That shielding will do absolutely nothing for line noise that is already on the line before the shield starts.

The shield will do absolutely nothing for radiated noise from the unscreened arc (Radio Frequency generator) feeding every frequency into every audio cable in your system.

You must minimise the loop area of every cable pair in your system to make it more resistant to the arc welder.
 
Have you spoken to your welding Neighbour?
Is his arc welder suppressed at his end?
Is his arc welder on the same phase as your house?
Contact the electricity supply company to move him to another phase.
Volunteer to buy the suppressor for his welder.

It is always better to suppress interference at source.

However the RFI from the arc can only be suppressed by surrounding him or you in a Faraday cage. Neither option is feasible.