[Closed] XX1 / X01 Cassette Wear

Just swapped the chain after around 600 miles of use from brand new on a x01 cassette, in 42t cog the chain is slipping under heavy load ( ie getting out the saddle to climb techy steep uphills) it's absolutly fine just sitting and spinning on road hills (even mega steep ones out the saddle) but put any major force onto it off road and it makes the dreaded noise like the chains about to snap!

Now when I swapped the chain for an identical xx1 chain I left two more links in as I felt this may have been just a tad to short first time! I had to adjust the b tension screw accordingly though to compensate the extra links

Absolutly all gears shift spot on, there's no hesitation in any gear change and it's crisp like new up and down the cassette, it doesn't slip or attempt to to do it climbing I'm any other gear apart from 42t

So just wondering is the cassette shot? If so that's shocking after 600 miles, or is it to do with the extra links maybe? Or will the chain wear into the 42t after a few rides?

Something tells me it's about the setup given it still shifts beautifully!