SLX framesets are definitely heavier than SL framests. I found that our with my Pinarello Montello.
I had a hard time approaching a sub 20 pound build weight on it, while my SL bikes easily hit weights of 19 pounds. I do feel thfeel Montello feels stiffer thsn my SL framed bikes. Not by much, but it is certainly there. It just feels more solid under me, than all of my non-SLX framed, steel bikes. Just like how an older Kercedes feels, compared to most other cars.