You are over thinking it. With an oil t stat(Which you should run on the pressure side), 90% of the oil should be bypassed back to your engine filling your cooler slowly. This will build oil pressure faster and avoid thermal shock. The windage experienced isn't worth debating at start up.
If you oversize your oil cooler, it can cause low oil temperature during street operation. Not a huge problem if you are running a real street oil with detergents. The oil t stat will still flow enough to cool your oil. My temps dropped substantially with my oil t stat and cooler. (Vs. my heat exchanger which you may consider?)I had way more metal flying around and likely put in a bigger cooler than you are planning. Moral of the story, you either need to install the right size cooler, or build a shroud. Rely on experience, there are plenty of LS engines in a dual purpose application, find out what size cooler at t stat they are running and the results.
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Todd
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