I run an x9 medium cage with the one-up. they want the the long cage if leaving 2x up front. and no need for the 'rad cage' unless 2x
i rarely use the 42t cog - it is extremely low. so low, that if you need acceleration to stay upright, it won't be there. extended climbs, without tech seems ok.
adding the larger cog, rather than smaller chainring maintains the top end speeds. you might able to spin out 30x11 rather quickly.
http://forums.mtbr.com/drivetrain-s...2t-conversion-oneup-vs-wolf-tooth-897313.html
There is one other piece of advice i received
per
@jimvreeland - nobody likes walking, don't change the bike, change your ability
Maybe that is why i haven't been using the 42t - but it is nice when i need it.