Wanted to update for anyone else that finds this:
My intent was to go Doug Thorley long tubes + MagnaFlow Y-pipe with cats, effectively maintaining the upstream and downstream sensor locations in relation to the cats and getting the performance boost without being harsh on the environment.
I just got off the phone with Lakewood Muffler and Brake and they refused to install this setup, as "moving the cats from the headers to the y-pipes is illegal" and I "would not pass emissions". Something about federal law and engine codes on the cats needing to match. Basically, they told me my only option was to reuse the factory cats from the headers, if they were good, or buy new OEM header(s) and cut the cat(s) out, if they were bad, to weld into the DT shorties.
This is confusing, since MagnaFlow claims on their website, even specifically for Colorado, that their y-pipe cats are EPA and CARB compliant, and even goes as far as to ask for your emissions tag code from your vehicle to make the product recommendation.
But, if you have good cats and want them to cut and weld to the DT shorties and install, I was quoted ~$700-$800.
Really want the long tubes.
Another auto repair shop in Lakewood I use for maintenance will install the long tubes, but they want friggin' $2200 to install them. I'm guessing they are just quoting book hours.
Still looking for a reasonable price on my intended setup..