skl1,
I think the 7x64 and its twin 7x65R are great cartridges. As John points out, they do virtually anything that a 270 WCF and 30/06 can do.
I also think hammers look nice on a DR. I shot skeet and clays with a double barrel 20 ga hammer gun. It takes a little practice (like anything else) but isn't really a handicap and adds fun to the shooting game.
Daryl pointed out that a SIACE sold fairly below expected price at auction. I have seen that happen before and also the opposite in which bidders drive the price skywards. I watched a gun sell yesterday got what I believe is twice its value. Auctions really show what the value of a specific item is at that moment with those active bidders, The sale price may be completely different on another day with different bidders.
Like John, I don't know much about SIACE but if you have researched them, and don't see negative comments, then that could be a great combination: DR & DB shotgun.