I took the mulch kit totally off, put on standard blades and the mower cut perfectly, just like my X530. I have several ideas why the cut is not good with the mulch kit but the bottom line is that I will NEVER put it back on again. Glad to discuss my findings with anyone interested.
I have the standard full-time mulch kit with the factory mulching blades on my X500/48" and I am extremely happy with it. I've been using it for 5 seasons now. No clumping, no missed grass and no visible clippings.
I just mowed again yesterday and the grass must have been 6-8 inches tall in places. I was cutting at 3 1/2 inches. It worked perfectly.
One thing that may effect the newer kit is due to the fact that you can switch between mulch and discharge, it uses more of a combo blade instead of a dedicated mulching blade. I suspect the combo blade doesn't do as well as the full time mulching-only blade.