I have a hard time believing the MS251 is being built on the MS211 platform, since that is also the MS171 platform... surely it would need some strengthening to handle the power of the 251.
as far as i know, there ist a new platform with 231, 251 and 241! just like 261, 271 and 291 one class higher. so we can hope, that the ms241 ist not only a bit lighter than a 261, anywhere beetween 211 and 261.