Richard,
You have the SnS easy spin SS grate right now correct? Is it still in good shape? If it is still working well for you I would really have to ask myself do I need it.
I have the two kettles, and one has the Killa Grilla grate and I love it(I also splurged and got one for my smoky joe and love it). The other kettle has the Aura Kettle Zone Cooking System with the SS grates which I equally love. The Aura grate does not have the beefiness of the Killa Grilla grate, but it does an amazing job and as nice as the Killa Grilla grate is I wouldn't say it is that it is a needed upgrade to the my other. I don't know how the SnS SS grate would compare to my Aura grate. If your were upgrading from the standard grate, I would think differently. Now if you have the money and want to spurge, you will not be disappointed by any means by the quality of the Killa Grilla, it is top notch, but if this is a push to cover cost, that is something you will have to decide.