Why should I switch out the matrix cube? I tested it in SimC and on AskMrRobot, and I saw a loss of about one hundred dps all the times I saw it in SimC. It would seem to me that BiS Gunslinger relics are the DG relic of DR and the matrix cube.

Then you're doing something wrong, since the DG Relic of Boundless Ages effectively gives 134 Power (when used on CD) which out weighs the 66 Cunning + 27 Crit that the Matrix Cube gives.