Concerning the HCF delete and K&N drop in:
I put a K&N drop in air filter in, couldn't really tell a performance increase but I do think it made the engine sound a bit louder.
A few thousand miles later did the HCF delete, didn't notice much of a difference there either--was hopeful, but didn't notice much. If anything, (and maybe I just wasn't paying attention prior to the HCF delete) the engine doesn't rev to 5,000 at WOT, instead it goes to maybe 4,800? Nit picking? possibly. Not really sure, but between the two mods I didn't really notice much difference--if anything I *think* more was gained by the K&N air filter but--again--don't know if that "gain" is only in engine sound. Who knows. For reference, my engine has 182,000 miles on it--filter and HCF delete were done within the last 5,000 miles. I'm hoping I'll get a more noticeable *gain* from a spark plug change.
One thing I can say--without a doubt--the interior LED swap is a GREAT first step towards customizing your 4Runner!