A 40T sounds good, but you may have to fit it on the outside of the crankarm to clear your chainstay. That will alter your chainline, probably for the worse when using low gears offroad. A steel ring weighs roughly twice an alloy one, so a steel 40T may be 150g but will last at least twice as long as even hardcoated alloy. Shimano do not offer a 40T for the regular groupos, so you may have to explore exotic alloy solutions such as TA or Middleburn. I think your SLX middle/outer crankarms are 4-bolt 104mm BCD.