I don't think the performance gain is worth the risk a of single chain, the twin chain is alot stronger and a safe gard if one breaks.
if you think about it...the fact that BMW have gone to the effort of developing a single row chain for the N4x series engine (we'll assume its safe) should be saying something.
And given it is a newer design, if it does fit?

it will also be most likely somewhat stronger than even our double row timing chains.
Don't forget that BMW was quite happy running the single row on the m30's and they even ran a single row chain on the euro version of the m88/3 in m635CSi's
