The consensus is that most people actually ride with cranks too long for them in the first place. A 175mm crank should really only be used by taller blokes, say 6ft or over (though it is of course dictated by the leg length rather than the total height of the person), at your height/leg length, you should probably be using 170mm cranks ideally anyway.
Why do bicycle manufacturers still supply bikes with cranks that are too long? I know people under 5ft6 and with short legs riding 175mm cranks! Well, it's probably cos a 175mm crank gives you a little extra leverage, making it "seem" easier to get up that big hill! The fact is, cranks that are too long knacker your knees. I'm 5ft11 with a 33" inside leg (so hardly short) and I don't like 175mm cranks, they make my knees hurt! I use 170's exclusively. Shorter cranks promote a better "spinning" technique, they're kinder on the knees and you get a touch more ground clearance. The only drawback is that most cranks are 175, so finding 170's at as good a price as 175's can be a bit tougher, but what price a new pair of knees!
This goes some way to explaining the crank length question too.
Regarding SLX cranks, BIG thumbs up from me! Have a set on one of my bikes, they're only about 20g heavier than XT, look cooler IMO, just as stiff, chainring shifting is superb (on my triple set anyway, not tried the double setup), basically I highly reccomend them. Certainly they're a big step up in quality from the last LX cranks.