I assume you mean 400% of the inlet cross sectional area.    CFM air in = CFM air out so 45 kts is about right given your numbers.  But I think the weak point in your assumptions is that the air entering the inlet is going at 180 kts. (or whatever your TAS is). There will likely be some external diffusion going on so that column of air velocity may not  = TAS.   The pressure required to force air through a typical oil cooler at 45 kts is probably a lot higher than you are likely to achieve.  Just a gut feel, I could be wrong.

Just as a reference point, my oil cooler face area to inlet area ratio is 650 %.  Inlet is ~16 sq.in. The radiator face to inlet ratio is 560 %.    Are these the right numbers to use?  Don't know yet but today's RV-8 flight gave me excellent oil & water temps (167 / 157) on a 55 deg day.  Still have lots of cooling outlet cleanup work to do and still flying without all my intersection fairings in place so I expect cooling to only get better. 

I cleaned the exterior of my O2 sensor with brake parts cleaner for today's flight but it was still dead.  Didn't try removing it to clean the hot end though.

Tracy (Whooo Hooo!  no smoke today.   I'm closer to taking this thing somewhere you can see it Ed!  )