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It is not an entire new bumper, in fact the rear bumper has been the same part on all iterations of the X150. The speed pack add-on is the part you see in white and at one point Mina did offer them. The difference between OEM and aftermarket is the material used. OEM is flexible urethane like the OEM bumper and aftermarket will be rigid FRP or GRP. The problem with rigid add-ons is they are very porous and can be tricky to fit properly and are typically best left to a specialist to prep, seal, paint and fit.
That's also where I purchased the R-S bits for the rear bumper and carbon fibre boot plinth at massive savings, Steve always seems to have quite the inventory. Reminds me of another UK supplier of genuine McLaren parts at huge savings over factory direct, Hybrid Performance LTD