Best in bag believes that I should replace a 445 item by a 430 item (nothing wrong until now) and also remove a ring so that I can reach the corruption limit that I imposed (39), dropping my item level by 29. Screenshot attached.!
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Thanks.
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From personal experience anzig vras corruption is pretty much the highest dps you can get on a feral druid unless you have a high dmg corruption that isnt tied to ilvl. You might end up wearing that anzig vra til you either get some better corruption, or a high anzig vra, as the ilvl of that item doesnt determine your dps so you could literally upgrade everything else and still probably favor that 430 staff over just about everything but a handful of pieces. as far as the ring, its likely saying the corruption from anzig vra outweighs the dmg increase from your ring which honestly wouldnt surprise me. anzig vras dmg has easily been 10-20% of my dmg based on procs on single target dps.
Go pick up a cheap ah ring or world quest ring without corruption, or cleanse the corruption on your ring?
Yeah you need another ring without corruption, or you can increase your corruption max to 59 – you’ll have to deal with an extra mechanic, but usually it’s not too bad.
I understand that (and thanks, all for the answers). It was more the algorithm I was challenging than anything else. I just did that, got myself a non-corrupted ring.
Always a good idea to query your Corruption thresholds before equipping new gear; your Corruption resistances are key to managing how much extra you’re happy to live with… zero extra Corruption is a very viable amount. 
@yellowfive in this case the optomiser wasn’t working correctly.
52,724 DPS BiB
54,798 DPS Equipped gear.
I realise that another ring has been obtained now so the unusual result has gone away but I thought running two sims was a sensible choice.