Yeah...I agree with Mr R.
I read through the moto-one stuff regularly to fuel my fantasies. If you read the exhaust thread carefully I think that Douglas851 is 5mm out on his sizings. What the moto-one article boils down to is:
888 Standard = 40mm = strangulation
ST4 Standard = 45mm = Good power & torque for not much cash
ST4 DP System = 50mm = No appreciable gain at the top end and a big hole in the torque curve
In summary it seems the 45mm system is the optimum unless you're gonna tune it further