According to what i have read the theory is when the walls and ports of the intake are mirror smooth the fuel in the air fuel mixture will adhere to the walls and wash along the walls. You want the fuel in the air not condensing along the walls.
When the walls are a 80 grit finish the surface is just rough enough that when the fuel touches the surface it will adhere but it will wash up to a "peak" on the 80 grit surface and remix with the air instead of washing along a smooth surface. This 80 grit surface
only applies to the surface downstream from the injector port when the fuel has been added. The surface before the fuel is added such in the intake manifold can be mirror smooth. Of course the exhaust port needs to be mirror smooth to help keep carbon deposits from forming.
Goto Standard Abrasives website and they have a great article on DIY head porting. It is also a good idea to have the exit of a port to be slightly smaller than the entrance of the port as the velocity of the gases will be faster. This is very important for n/a vehicles and very important in exhaust ports. This is why i do not port match the exhaust ports to the exhaust manifold and the extra lip help to be a reversion dam which i would think would be helpful in a log type manifold on our cars.
Goto
http://www.sa-motorsports.com/diyport.aspx
for the SA article and this is a quote from that article.
Once you have worked the port entry with 40-grit, switch to the large, straight 80-grit cartridge roll (part no. 263163). The 80-grit gives you the smooth, but not polished, surface that is correct for intake ports. Once you finish the 80-grit step, stop, remove your gloves and feel the surface. When your tactile "QC checker" (your finger) senses the right finish on all the port walls, you're done.
The mirror finish in the intake manifold is fine but in the head from the injector to the valve seat (bowl area) needs to be 80 grit finish IMO.