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Both are of course drop-in; not fitted.
The chambers are high strength stainless steel; the tubes aircraft aluminum. Early on there were reports of the 12 to 20g tubes cracking at the extractor cut-out. Have not seen any reports recently.
Briley at one point & maybe now recommended only standard target loads to be used with the tubes.

Your gun barrel simply acts as a carrier barrel; no pressure transmitted to the damascus tubes. I've seen reports of guys using the tubes with cracked damascus barrels.

The Companion tubes come with flush choke tubes.
The Sidekick 12-20 and 12-28 seem to keep the original choke %; the .410 maybe less so.

I had 12 - 28g Companion tubes which worked fine except as drop-ins they do not extend all the way to the muzzle, and there is a significant deposit of hard burnt power/plastic residue that builds up just past the end of the tube in the bore. That was a problem with the old Savage 410ers, and definitely occurs with the shorter Sidekicks also.
SO they need to be removed after shooting and that ridge scrubbed off.
Addendum: I wonder if one could purchase Briley extended choke tubes that would exit past the muzzle? Might not work if the original barrel is full choke.

re: working in 2 1/2" chambers. Small image but it shows the profile of the external taper 12 to 20g Sidekick, and I believe it is for a 3" chamber. It is possible Briley could custom grind the chamber, at a custom cost. If you talk with Briley please let us know.