My level 15 khajiit Montes has become an assassin. I wear all brotherhood shrouded armor EXCEPT for the cowl, where instead I have the hood which gives a 35% boost to sneaking. I also have the nightweaver's band with a 15% boost to sneaking.

Will all of my sneak bonuses add up? (40+35+15=90% better sneak)

No, you get 40 + 50% out of 40.

I believe percentage bonuses are multiplicative.