Custom AFM screw. $15-20 in qty of 50
Home Depot mod - $10
Better Air/Fuel mod tuning starts at $400 (E-Manage)
then starts to move up.
There is a lot of good information here, and I too went this route. Here are some very important things to note. YES the Lexus AFM is bigger and can MEASURE higher flows than the stock AFM. It DOES need tuning, but here are a few signs of trouble...
1. The Lexus AFM upgrade is literally a 5 minute scewdriver physical install, unless you use the EXISTING air snorkel from the old AFM. If you do - you will notice that the LEXUS afm bolts have wide shoulders at the end, requiring you to drill out larger receiver holes on the snorkel. Still not a problem - takes five more minutes.
2. With the LEXUS AFM installed, start the engine when COLD with NO THROTTLE opening at all - see if it idles, or if it stalls. IF it refuses to idle, or idles very roughly-threatening to stall, then your fuel-system is running too lean with the 550's installed, and you need to find out why: (Injectors not at spec / Low fuel-pressure / other).
I ignored this myself, and upped the idle screw setting, and she ran like a beast - I ran it like this for over a year, until I burned a rear piston - from a too lean mixture. Get this - my Tru-Boost read 29.3 PSI of boost when the piston burned, and it did not damage the face of the piston, it burned around the top ring and thru the side of it!! Lesson Learned!! CHECK YOUR MIXTURE with a competent measuring device after this upgrade. My situation was OLD 550's (one running at way below it's flow rating). Leaner means hotter - more power - yes, but can eat a piston, and/or head gasket.
Now there are other things I am doing to permit those levels of boost without detonation, or pre-ignition (trade secrets), but I was boosting to over 25 PSI daily on pump gas - you guys will be way lower than that, but still - have your mixture levels professionally checked...
That is my 2c worth of scar tissue for you.