I emailed Mintex about trying to get some 1144's for my Focus RS as they are currently out of stock of 1144's for the FRS, they suggested i use the xtreme pads and when i asked what the differences were between them i was told this :

"we have four materials in the Xtreme range with number five under development, these are matched via vehicle performance and weight. The material assigned to your Focus RS is X1 which has the initial performance of M1144 at 0.42 but with the benefits of noise reducing OE quality foil shims, lighter coloured dust deposits and a cheaper retail price. M1144 will continue in production for some time yet and may be further developed into a more focused track day material."

So yes the xtreme pad material is an update of the 1144 compound and has the same coeff rating as the 1144's