What am i missing? i can but a complete shimano 11 speedd shifter, cassette (11-46) & derailleur for less that the price of replacing my worn sram x1 cassette.
The gearing calculator gives me a lower bottom gear and a slightly less higher gear, so no problems there.
Will my absolute black oval narrow/wide work or is a shimano 11 speed chain different?
I havent used shimano for years.

You’ll be needing a new free-hub as Shimano don’t work on SRAM free-hubs

Steve_b77 is right. But apart from that, you’re not missing anything. Go for it.

Added bonus with Shimano is that the cassette can be re-purposed as a boat anchor when it’s done on the bike.

Seriously though, you can use a Shimano cassette with your existing gears to make it even cheaper, bar the freehub replacement as was pointed out.

Sram XD driver cassettes almost need a kidney sold to buy (well ~£80), Shimano 105 11-32 can be had for ~£30.

Yep I remembered the freehub after I posted, still works out cheaper than a sram cassette!
My derailleur has loads of sideways play on the frame bolt and I’m not convinced the shifter is back together properly after I accidentally took it apart whilst changing a cable, along with a worn cassette all leads to difficulty in changing down into the 11th cog.
As long as I can retain my narrow wide oval thats fine, I will replace the whole lot.

I’ve heard way too many stories of the SRAM driver end cap snapping, rendering cassette replacement even more expensive than it currently is.

For that reason, I’ll stick with Shimano 11 speed, certainly until SRAM can market a cassette which matches XT for quality and price.