SERT installed with canned map

Dealers often install maps based on known combinations without checking against a dyno. I wouldn't worry to omuch as long as you aren't running lean at idle, midrange, and WOT. I would keep an eye on the plugs to make sure they keep a nice tan appearance (start with fresh plugs).

It all depends how bad the decel popping is but I don't think mild popping will cause any damage. The decel popping itself isn't all that damaging, its the associated causes and conditions that may lead to other problems. One way to help reduce popping is to install a set of AR cones between the head pipes and heads. This can sometimes cut back on the reversion which worsens the popping. Also double check your whole exhaust system for leaks. Cool air drawn back into the system will increase this condition. With the exhaust cool, spray soapy water onto the various connecting points while at idle. If you see any bubbling (not to be confused with sizzling on hot pipes) you have a leak.

Personally I've learned to live with mild decel popping or crackling. It can be a fact of life if you run aftermarket pipes such as duals that don't have a cross-over or regular baffles.

Yes, you can still use them with Twin Cams even though their smaller exhaust port is supposed to act as an anti-reversion barrier. Depending on the pipes, you may need to grind down the flange edges of the cones for a better fit. There are a few brands out there but they are all pretty much the same. Some call them AR cones, Torque cones, or Power Cones.
I've seen them at many local dealer, J&P, and even found this pair on ebay.
They may be listed for Evo but they'll fit a TC88/95 also.