I've seen this written in several threads around the web, some by EE's. Some say double up the copper thickness on your connecting cable, which acts as a spur, as the in wall cable is usually a ring circuit, so make the spur thicker to compensate. Strip the twin and earth conductors out of the sheath and make a twisted pair out of the L and N and run an insulated earth along side the pair. Cover in an expanded woven jacket and you have a great lead.
I wouldn't worry too much about what manufacturers say, their recommendations are more about not getting sued if things go wrong, so "change nothing" is the order of the day. Flexible leads are meant for a lot of use/abuse/coiling/uncoiling etc like hair driers/power tools etc. Your audio gear is more of a fixed installation, I only move mine a few times a year and the mains cables tend to stay put, its the gear I move, so not a lot of flexing involved. You can stick your tongue in a live socket if you want, it's not illegal, just not advised.
I will get round to making a lead like this one day, using a straight exit industrial three pin plug one end and a locking IEC kettle type connector on the other. The Hifi plugs offer nothing other than the ability to accept thicker cable jackets and are a rip off. Industrial plugs offer the same, can be found with Silver plated pins if you want to go that route, and are made from the same materials, so you are paying for the myth that the "hifi" versions are special. I personally only use a three pin British safety plug in one place in my system, and that's where the single mains lead connects between a wall socket and my mains distribution unit. this is the only fused plug in the chain and its there to protect the lead from catching fire and also the ring main, in the unlikely event any of my leads get tapped in a door etc and give a direct short. All the leads from the distribution unit to my gear have IEC kettle type connectors each end, so avoiding unnecessary multiple fuses/connections. I make the IEC leads up myself from decent thickness copper and have no issues with what I hear. The only negative thing I experienced was when I had a filter in my distribution unit, it sapped the life out of my amps, when I removed the cheap and nasty filter and made all the connections straight through, point to point, the problem went away.
It's a simple approach but works for me, that is the important thing, if you are not experiencing any mains issues, don't go chasing rainbows and stick with what you are doing.