Only things I've found (I'm in England and have an annual emissions test)

Retard the timing from 34 idle to 20. Helps heat up the O2's and cats.
Increase the idle to as fast as allowable 800 upto 1100

I have a single 3" pipe & cat back to the rear muffler and 2x 3" outlets. I've found sealing one up and reducing the one with the sniffer pipe in will allow the CL to target properly.

On a normal test Lamdda shows up as lean, over 1.03 (limit), but with high HC. Sealing the tail pipes usually pulls it down.
I think the reversion sucks extra air into the pipes & the low flow rate DOES NOT work with the short test pipes they try & stick in.
This also makes the car very hot.

HTH
Mitch