Mike, good questions, let me start by saying, on any given day a non lambda CAN burn as well as a lambda, however not on a repeatable consistent basis.
If you were able to identify, monitor and manipulate the known parameters a lambda boiler does, [ o2, co2 flue temp] on a real time basis you could come close. Your p/s air setting is a best case approximation for a batch burn that has consistently changing requirements. Your fixed setting has the capability to burn efficiently for one of the three burn stages if your wood pile has a consistent moisture content. I am referring to startup, middle of burn and ending, most fixed settings are tuned to the middle or longest part of the burn. This strategy works very well if you know your boiler's burn charachteristics and can change the settings if wood moisture content changes drastically. These are known documented facts.
I think the confusion stems from how much of a advantage is lambda really? The answer is how far into it do you and the boiler mfgr want to go for a acceptable effiency increase and still be marketable. Each manufacturer creates a in there opinion a efficient boiler housing, [for water hx] and wood combustor [ transfer wood to heat] and a control package. With a highly competetive boiler market , lambda and non, alot of effort goes into the boiler housing and combustor design. It is probably safe to say most are very efficient and the playing field realitively even in this aspect, this is drilled into us with glossy multicolored boiler porn.
Really very little info is given about the control package, other than it will burn your batch efficiently. Here lies the real difference in these boilers [ lambda] and how much effort is viable, for resale. I am going to list the differences of the degrees of lambda control, each with a different cost and effectiveness of that control, I am not able to transfer that to overall boiler effiency, and likely at the end of the day the manufacturers will want to tell you theirs is the best. All lambda systems start with a o2 sensor and a thermocouple in the flue, that feedback goes to a controller that outputs a signal to the final control element[ damper] in a draft induced system, to keep on topic I will focus on this scenario, not a fan forced draft system.
Simply the o2/ temp feedback provides the current combustion state, forwarded to the controller to calculate a correction for the damper to make, and is reflected in the next o2/ temp sampling sent to the controller. The controller is trying to maintain a predetermined setpoint , [optimum effiency for wood combustion] each mfgr has a one set based on their combustor design, usally can be changed. With this strategy, sensitivity of the feedback, controller and dampers could be thought to be required for success, again this is for you to decide. The least sensitive would be a stepper motor with a large step change in damper position, smaller step changes, and finally a modulating actuator would be the most sensitive with respect to controller output. The controller driving the stepper or modulating actuator can be a off the shelf $30 rudimentary single input/ output to one with a propriertary designed algorithim for multiple inputs/ outputs at $5000 or more. With added sensitivity comes calibration, maintainence and other costs.
In my opinion a well tuned relatively sensitive lambda will consistently outperform a non lambda, however each lambda mfgr has to figure if they are going to hide behind the ''lambda badge'' or exceed it, in the end there is a boiler for every buyer, you choose!