Just updating in case someone needs this info in the future:
I measured a stock intake as best I could without disassembling it too much. it's somewhere around the 2.75" mark (+/- 1/16") However, I found with a sleeve this size on my IS400, I was running air fuel ratios around 11.7:1. Way too rich for a NA engine. 3.0" tube got me around 12.4:1 and 3.5" got 13.5:1
This was done with a computer reset each time to help reduce the adaptive nature of the OBD II computer. Now that long term trims are adjusted, I'm sitting right at 13.0:1, which feels and sounds about right. (nice crisp note with out any surging)
-B