True, but only if the car is 4 years old or less. Older, and option prices are irrelevant.
M-Sport Plus is an option pack with a price, rather than a trim level in itself. Revenue add a diminishing percentage of this price to the OMSP, the exact % being determined by the age of the car. Beyond 4 years old the value of options is irrelevant and is disregarded as regards OMSP. The price they use for a computation is most likely the price declared locally by the distributor for the same option pack, if its available on equivalent cars locally. If its not, I image that they use some common sense in grossing up the price of the pack in the market of original sale, generally UK.
I think the UK price of an M-Sport Plus pack on an F30 a couple of years ago was around stg£2.5k.