Question 12-Pin GPU Cable - Will it work with my PSU ?

In the page you linked, the cable says:
The CORSAIR 12 pin GPU Power Adapter Cable connects any Type 3 or Type 4 CORSAIR modular power supply

Down at the bottom has a link to the compatible power supplies. If I read that correctly, your RMx is compatible with type 3 and type 4.
 

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