Err how would a decat cause the engine to run too lean? Unless there is something different with the K-Series engines to B-Series and H-Series and D-Series?
Your reducing the work the engine has to do to pump out the exhaust gases as removing a restriction. So there is a performance gain. If you can remap the ECU the gains are even better.
Decats will ALWAYS make a car run lean as well as removing the restriction that's why if makes the power it does.
Lean makes power BUT causes det.
A car runs in what is called "Closed loop" all the time you are part throttle. This means it is always measuring the air fuel ratio for optimum 14.7AFR for economy and self adjusting the fuelling accordingly. You will only feel the decat or lack of restriction caused by the cat at this point as the engine will be a little more responsive.
Now go full throttle. The car then comes out of closed loop are runs totally off the fuel and ignition maps and no longer auto-adjusts. Now you have removed a pretty big restriction the map that was setup for a stock intake, cat and manifold for correct "Safe" fuelling no longer has the restriction and if you have other mods also you will find it flows more air....as you rightly point out, however the cars ECU is still only delivering the correct fuel for a stock car....with restrictions....Hence it runs lean.
Hope this explains it a little better.
I tried a decat even with a Hondata ecu and although it made good midrange gains it actually lost me power at top end. I removed it and got the top end power back.
What was seen on several K20 decatted EP3's was you actually lost power over stock, but if you mapped to it with a mappable ECU you culd gain upto 10bhp.
Basically as has been said, it will work but only if your mapping a car. If your not it will do no good atall.