Buying a new Cadillac DTS - best options or aftermarket?

I'm a big guy with more of my height in the body, not the legs. I would love to have the STS-V, but I need the DTS for the shoulder room.
What options do I need to consider to get the best performance DTS out
of the showroom?
Or, are there aftermarket accessories, kits, or especially BUILD shops that specialize in making a performance DTS?
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