Your client that is an 'older gentleman'--how old is he? And he really rides outdoors in the the wind at -15 deg. C? That is impressive. But is this a 5 min ride or 3 hr ride? I have ridden for years in snow in temperatures approaching that, but probably more often like minus 10-12 deg. C (but also in good winds which drop the perceived temperature lower). Do you? Have you tried the sidi's yourself? I notice around where I live that very few folks (practically zero) actually go outside on bikes in these kind of temperatures.
Yes, my method with the booties may be a bit of work, but 'outlast' fabric or no 'outlast' fabric, I think that the sidi's will have shortcomings in very cold temperatures. One problem that one quickly discovers in extremely cold temperatures is that even if you have a shoe
upper that is very, very warm (perhaps like on the sidi's), the cold can be rapidly conducted by the outsole, the pedal, the spd cleat, and the metal shank that holds the cleat--as metal does this very well. This is even more the case if you have snow clinging to the bottom of your shoe. For this reason, some folks avoid using cleated shoes entirely (they use toeclips) in such conditions.
Another thing, the sidi's are quite expensive as you have to buy another pair of shoes AND you also have to buy regular booties to put over
them. My 'difficult' method costs around $35 (US) and you can use your original pair of off-road bike shoes. And you can adjust the layers to suit the conditons of the day.
Where would you want to swim ?