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Three words for porosity issues... Vacuum Casting Impregnation. The OEMs use it quite often on their aluminum castings. I used it on my Dove 2x4 Tunnel Wedge intake. Dove pretty much invented casting porosity, lol...

The VCI process is a wondrous thing. The item (block, intake manifold, alloy trans/differential housing etc) is placed in a vacuum chamber and the chamber is drawn down to about 50 microns (29.86" of vacuum or there abouts). The vacuum is then broke with a highly viscous, permeable and hardening fluid. Since every minute and not so minute pour is under vacuum too the fluid fills the voids completely. The fluid after hardening is completely durable and good to any temperature that the aluminum itself could stand. It will last as long as the part does.

On the down side, all machining must be accomplished prior to the process. For a block the process would be around $200. Volume draws down the costs too.

Just a technical tidbit that may or may not be useful.