The use of SPX for frame sizes around 60cm and bigger doesn't surprise me. SLX gets too whippy on big frames with the standard diameter tubes.

Both SLX and SPX only have the helical reinforcements in the butts, so most of the length of the top tubes and down tubes (the non-butted centers) don't have the reinforcements.

I could be mistaken, but I believe the only tubeset with the helical reinforcements over the whole length of the tubes is TSX.