Btw, these SSD chips uses mSATA interface so I'd expect it has faster read speed than the ordinary SSD we can buy in the shops (which it does have) and normal write speed. So the interface channel is not a problem here. (mSATA is not an established standard , therefore it requires a "matching" motherboard socket to work or it'll run at SATAII speed, that's why we don't commonly see it in retail market)
if you compare the performance between other mSATA models (all of them have size and low power requirement), Sandisk X110 128GB (427/273 MB/s), Intel 525 Series 240GB (508/262 MB/s), Plextor M5M 256GB(504/414 MB/s) all can achieve over 200MB/s write speed.
At the price of Surface Book, I'd think the choice of hardware would be at least above average.