BIG Turbo build planned - turbo guys!

It is just that the porting does not do a whole lot on most engines for airflow when you are putting any significant level of boost on the intake. You are not struggling to increase airflow within the limit of just 14.7 psi of maximum atmosperic pressure like in a normally aspirated engine; you can step up boost pressure with the right turbo to 2 to 3 times normal atmosperic pressure and overcome a lot of restriction; in that way, turbos are easy.

You can push 80 HP per cylinder of boosted airflow through most stock heads and more through some like a good hemi head; 2-2.5 HP per CI is easy to get with all sorts of engines if you can make it live. The turbo system airflow is mainly set by the smallest restrictions, which is the throttle plate (obviously I guess!) and then the turbo impeller. Valves are down the line in importance (which is partly related to the fact that turbo cams are not nearly as open as normally aspirated cams) and then porting at/near the bottom of the list. You'll get more from a free flowing exhaust which helps the exhaust side restriction, and as large an intercooler as you can get, which keeps the charge cooler and compressed and the density up. Impeller design will have a lot to do with off-idle response; the guys that make these all day long have a lot of tricks.

For 500 HP, maybe the porting on THIS head is needed..... but for a 225 ci, 6 cylinder, it would not normally be needed with enough boost pressure. I would try it without first unless someone has already established the need, and focus on keeping the thing alive at 300-350 HP and then work my way up in pressure. For 500 HP, I think (know) there are a lot more thing that will need attention than the porting.

And, OBTW, on the AL rods.....steel (and some irons) have a property that aluminum does not possess. If the distortion/load is kept below a certain level, steel can be loaded and unloaded an infinite amount of times; it never has a life limit. Aluminun never can do that; there is always a finite number of load cycles on any Al part. You can increase the number of Al load cycles but never make it infinite. So there is some truth in the limited life of Al rods. It probably has been made better by BME, but there is no such thing as infinite load cycle Al. (Otherwise perfectly good airplanes are retired after so many flight hours due to this factor.) Hence, the article cited is likely truthful in saying the rods will live on the street for a couple hundred kmiles. In a 500 HP slant 6, however, the loads are going to be pretty high. I kinda epxect the OP to be leaning on this engine....a LOT.