Are billet con rods stonger than forged steel ones that have been machined after they are formed?

Yes. Billet is always stronger then forged. Believe it or not Metal has a grain to it. With billet steel the grain is more linear due to the extrusion proecess and therefore is stonger.

Before dumping a bunch of money into a set of rods look at what HP engine will be making and choose a rod to fill those HP requirements. Most 4340 steel street style rods will handle up to 500HP. The stock rods if reworked with stronger bolts should be able to hold up to a maximum of 400Hp, but no more. Do some shopping, as reworking th stock rods can cost more then some of the steel rods being offered from other manufacturers, plus you get the benefits of the aftermarket rods having good bolts, steel forgings, oversized piston pins, better balance, less stress risers, etc. Plus much stronger.

I thought forged and CNC'd(stress relief) was stronger. (You don't cut the steel grain)
It's like growing a tree to suit you needs instead of carving a piece out. Please correct me if I'm wrong.

A billet rod is a CNC process. The material is a solid piece of bilet steel and the CNC cuts out the rod. The grain of the material is linear throught the whole piece. A forged rod is a piece that is close to what the finish piece will be and then a mill or cnc comes in and removes a minimal amount of material to create the final out come. What makes the grain different is the material has to flow to the shap a forging rather then being extruded. If I remember correctly.

Thanks