This is NOT an official method. I been working on this method for few days and I manage to get something near. They are not 100% Accurate.
Check the downloadable file for substance painter export preset before going into steps.
In substance painter, you will need to setup specular map.
If your material layer has metalness level up to 1 max, you will need to set specular color to same as what base color has. If there are no metalness, leave the color white.
In RPRUberMaterial, your nodes will need to be setup like this. You will be using gammacorrect node for roughness.
Specular->Reflect Color|Reflect Weight
For the connection between file and gammacorrect node, connect File Node Out Alpha to Gammacorrect node Value X. and connect Out Value X to RPRUberMaterial. There are no need to change any value in gammacorrect node.
In Render Settings, Effects Tab, under tone mapping, enable Apply Gamma To Maya Views and change your setting to these, with this, you have no need to disable SRGB Gamma mode, be sure to change display gamma to 2 before attempting Batch Render/Render Sequence.
That would be all, only difficult part/most time you will be using is on Substance Painter, As the render result does not show how Specular map looks like, you will need to export multiple times to get to what it suppose to look like.
Here are the comparison render images:
Left:Radeon ProRender in Maya
Right:Substance Painter Iray
As I wrote before, this is NOT an official method.I have uploaded export preset for Substance Painter for your use.
Feel Free to post your methods or Q/A