The Lex/550 mod isn't anything that's going to kill your car or leave you running so rich that gasoline is pouring out of your tailpipe. It's a cheap, crude mod and there are better options which cost little more (SAFC IIs are pretty damned cheap these days), provide more control and better power.
Ok, a Lex AFM housing may be used to limit restriction on any setup as you're reducing a bottleneck. The Lex/550 setup works under the gross assumption that a 25% increase in air needs a 25% increase in fuel... The problem here is that neither flows with linearity as duty cycle/volume increases. Most people who are trying to get the most they can out of their 550s are using a fuel controller and the Lex AFM is optional... In this case it provides some very subtle benefit in throttle responsiveness but it's not a lot (it is a little more noticeable with a big turbo but if you're running that large of a turbo these days you should be running a better EMS anyway). The stock intercooler and IC plumbing are just as much, if not more of a restriction than the stock AFM housing.
A peak/hold meter on the Vf signal would be useless. It's a dynamic output which is constantly fluctuating. The problem is the minute you let off the throttle it goes 0v and the minute you go 70% throttle or more it'll go ~4.8v, the rest of the time it bounces all over and you need to average it to get an overall trend. I think it's just a worthless light show unless you have it connected to an oscilloscope or a datalogger and the car is running in a controlled environment (just sitting out of gear with the engine held at certain RPM or on a dyno). With a data logger you have all the other operational stats to compare to the Vf readings but at that point you've already got more info than you need to determine the cause of a problem (because the basic feedback mode of Vf doesn't *really* tell you a thing). Reading it with a sluggish, analog multimeter will give you an averaged reading too (which the TSRM specifies). Vf is the most overrated and misunderstood diagnostic device on the MKIII. It is NOT an A/F indicator, it is NOT an injector or AFM diagnostic and it is NOT something you can use to tune your fuel controller. With a special TCCS service manual from Toyota and an oscilloscope it does tell you a LOT about the ECU's operation at any given point but I feel it's not helpful while it's actually in a car and not connected to a test bed. Ok, I'm done complaining about this particular system... /RANT
Wideband controllers run their own diagnostics on the sensor whenever they start up during the warmup cycle. They also usually have an 'emulated' narrowband output which you can connect in place for the signal line for the stock narrowband sensor so you may delete that unit. So you've got a diagnostic system, a sensor which accurately measures your fuel mixture (as opposed to the guesswork done by a narrowband), an output to simplify and supply your ECU with the older signal type it expects and you can change the signal range of the sensor's output and use it to adjust the fueling ( <-- I DO NOT RECOMMEND THIS). You can also recalibrate some wideband controllers to account for an aging sensor (Innovate Motorsports' controllers are usually ranked as the best when it comes to this.).
Air flow meter conversion is a separate entity. Some advanced piggy back's have this bundled in but there are units available which do nothing but convert from one type of meter to another. Yes, regardless of what system you convert to fuel cut will remain at relatively the same place unless fueling is changed too. Change the fueling you change the air flow scaling and then you change the fuel cut level. There are piggy backs for changing or eliminating the fuel cut level too and again some of the advanced ECU piggy backs have this bundled in, but it's still controlled separately of the air flow conversion functions.