What are your experiences with upgrading power cords on KEF LS50 wireless?

As one who primarily believes that power cables are power cables, I also know that isn't exactly true.  For example, there are 18 AWG power cables and there are 10 AWG power cables; there are cables with shielding and cables without.

You're right, there is a difference in the gauge. All else equal, the thicker the gauge the higher the amperage the cable can safely handle. A variant of Ohm's law is Power = Voltage X Current. If you're in North America where we use 110 volt AC wall outlets and your KEF LS50w is rated for 230 watts max, then

In North America:
230 W = 110 V x ?? Amps
230/110 = 2.09 amps

So you want a cable rated to handle 2.09 amps + a safety margin. 

In Japan:
230 W / 100 V = 2.3 amps

Europe:
230 W / 220 V = 1.04 amps

And in that case, this 18AWG cable will be extremely safe to use as it's rated for 10 amps, far beyond what the LS50w will ever draw:

Monoprice Power Cord - NEMA 5-15P to IEC 60320 C13, 18AWG, 10A/1250W, 125V, 3-Prong, Black, 3ft - Monoprice.com

I both own the original KEF LS50w and am an electrical engineer.

The only benefit the KEF cable has over the monoprice one is that it's made from that nice, soft, pliable sheathing. I've never figured out where to buy that stuff.

I'd add that I replace the networking cable the LS50Ws came with for more distance with this:

Monoprice Cat6 PoE Ethernet Patch Cable - 600V, Shielded RJ45, Solid, 550MHz, STP (U/FTP), 24AWG, 1ft, Black - Monoprice.com

which is identical to the KEF cable other than again that nice soft, pliable sheath.