Like he said I keep reading its not the actual design of the nissan rods as they are quite strong, but simply the small imperfections in there. As most will know a tiny crack or tear can turn into a large one under stress in any material. Higher HP, and/or detonation is going to do that. Shot peening, or blasting the surface with tiny beads or "shots" clears and smooths the surface, hopefully reducing any cracks or imperfections and the probability i guess of them spreading, and then failing. Hence I guess giving a higher tollerance for error in the rest of the engine.
If you can't afford the rest not sure whether it really makes too much sense just adding forged pistons on their own... as it seems like you'd be taking other shortcuts there too to save costs?