Hi typesomethingwitty,
Welcome back!
It’s hard for me to believe as well that the Overnight Mattress saga was 6 months ago … and I didn’t even get married in the meantime so I have no “excuse” for time going by so quickly (except getting older of course) 
The main benefit of a thicker base layer (with the same material) would be more “gradual” compression and that it can be more adaptable to different sleeping positions for heavier weights. If you imagine for example a 3" layer on the floor … it will go from as soft as possible (very light compression) to as firm as it can go in the space of 3" (or a little less) before it can’t compress any more. A 6" layer will have much more room to absorb the pressure more gradually till it reaches maximum firmness and will also spread the weight around more (as you sink in more it will start to bear weight under some of the more recessed areas of the body that are compressing the foam) so it won’t reach the bottoming out point or become as firm with compression. This can be valuable for heavier weights or larger body sizes and shapes that sleep in several positions where the mattress needs to adapt to different body profiles without such a sudden shift from soft to firm.
Much of the “feel” of a mattress comes from the comfort layers so it won’t make as big a difference in “feel” although it will “help” the pressure relieving layers more.
As you mentioned … it also has the benefit of being able to exchange the comfort layer if necessary. One of the members here also mentioned today (which I didn’t know) that you can also choose separate firmness levels of the top layer on each side of a King Size.
The costs involved in a layer exchange are in post #14 here.
So the 5.5" base would be “sufficient” but IMO the extra thickness would be a benefit for your weight in terms of performance … regardless of the exchange options.
Phoenix