Nice looking bike, how do you like the 4.2s? I ride some really rocky (huge boulders) very long rock gardens along with the usual roots & log piles, if I thought the 4.2s would hold up I'd like to go that way instead of the 1750s which are the 5.1s. I don't do any drops to speak of and no jumping.
I ride the same, the 4.2d's will certainly do just fine for you. For me they work just fine. These wheels are the second ones in row for me with the same rims, and honestly speeking I didn't consider any other when ordering the I9 wheelset. The 1750s are downhill oriented enduro wheels and you see them even on dh bikes.