I have a dyno session scheduled for December 30th, when I intend to look into that very same question. I'll have wide-band oxygen sensors in both the straight and the merge collectors to see how they effect the mixture.
When I bought the merge collectors at a flea market, I bumped into Super Gas racer Tom Goldman, who told me to expect to richen 3 steps, just as you have found.
If you want to find out my results, e-mail me at
tony@brentwoodmanagementcorp.com, on or after the 30th.