Back to your intake size questions:
It'* about flow dynamics. Your MAF sensor is the critical link. If you flow (for example) 50 CFM'* through your throttle plate with your existing intake pipe, and the flow (for now) is restricted to that at WOT by your stock airbox (which is your CURRENT restriction judging from our experience here), and you change to a different intake with less restriction, these are your possibilities:
1. 4" feed from a good filter:
Your engine, being nothing but an air pump in reality, will pull air to a certain amount. Let'* say (non-reality) that the 50cfm you were pulling increases to 65cfm. The problem is that the 4" pipe is too large, or overly large considering what the 65cfm NEEDS. The air isn't laminar. It swirls and eddies around waiting for a place to go. But your MAF sensor needs laminar flow to get an accurate reading. You'll actually read less air than actually exists, running over-rich, and hurting performance.
2. Drop that pipe size down to 3". Now you're closer to what you probably need, but the air flow is better suited for the volume of the pipe, increasing velocity, and causing laminar flow. Your MAF sensor reads more accurately. You get the proper AFR, increasing performance over what you had in option 1, and over what stock was.
3. You do a 3.5" FWI. MY THEORY is that based on your mostly-stock engine, you'll be somewhere between options 1 and 2, but still better off than the restrictive stock setup in terms of flow AND intake air temps. Both affect power. Colder air allows the PCM to add more timing.
ADTR did some testing long ago and IIRC, they tested 3", 3.25", and 3.5" intake pipes for the purposes of comparing flow at a given CFM based on what most of our cars were flowing 4 or 5 years ago (mostly stock). They saw a drop in CFM'* with the 3.5" pipe. I have the first 2nd pipe produced (it'* now on my 95 L67, as I went to a 3.5 FWI on the 93 this year). PeterG22000 is now running Hector'* original pipe from his mildly-modded L36. The funny thing was that the 3.5" pipe actually was determined to be slightly worse than the 3" with a stock L36 flow number induced.
Why? Because of the laminar flow variable, and the need for the MAF to accurately detect the amount of incoming air.
Will a 3.5" INTENSE FWI hurt you? No, you'll gain. Maybe not as much as you would with a slightly smaller pipe, but you will gain. And it sets you up nicely for future mods.
And don't forget that the length of the pipe creates a pressure drop. Friction from the inner wall....blah blah blah. You know where I'm going with that. But this is why the pipe is larger than the throttle plate. The larger diameter makes up for the friction loss from the length of the pipe.