Since Substance Painter supports and actually has it's strength in PBR materials that use a metallic and roughness model instead of specular, I think one should rather use also use this for NL2 then. NL2 Materials at least directly support the usage of metallic and roughness maps / channels since NL2.5.4 although it's not used for "true" PBR shading as far as I know.
An updated material tutorial would be great I guess, in a nutshell it can be used like this (it can be at the time I'm writing this only be set up using custom shader code):
Simple setup:
result = tex0; //base color texture
metallic = tex1; //metallic texture
roughness = tex2; // roughness texture
What I actually prefer, but this requires setting up an export profile for SP or combining the channels manually:
I call this compact setup:
result.rgb = tex0; //base color from tex0's rgb channels
metallic = tex0.a; //metallic information in tex0's alpha channel
roughness = tex1.a; //roughness in tex1's alpha channel (tex1 rgb is the normal map)