Rumor has it the 858 Catalyst engine package (engine, exhaust, intake, mounts, etc) is 3 lbs lighter than the 600 catalyst. Snowest even mentions it in one of their pod casts. The 600 does use a 800 crank, and their is savings in the integrated engine mounts built into the bottom end case of the 858 versus the plate system on the 600. This is how the 858 sits even lower in the chassis compared to the 600.I hear people say that 600 catalyst feels so light. Well guess what, so does 600 summit and so does 650 pol feel too.
858 will feel significantly heavier than 600. Thats just the way it goes.