If you ever read the dynamics of air flow, Turbulence is a very small resistance to flow. The biggest restriction to flow is the size of the hole it is passing through. This is a scientific law, so fact. If you decrease the size of the hole for air to flow through you increase the resistance to air flow by an exponent of 4. Turbulence does increase resistance, but it is less than a tenth compared to a change in diameter.

 

Just get the eBay unit, it made 15WHP on a Buschur car, he now sells them or used to sell them.

 

What about quaility of "ebay products"?

I've chosen split design from U-R.
It is on the way to me so I can not tell from my experience but the split design gave about 150 degC lower EGT in my friend's car.