Regards to displacement: All depends on what you're trying to accomplish. Seen 4.6l put out anywhere from 300hp all the way to 3000hp. Bigger the engine heavier moving parts...
My car has laid down 822rwhp and still has plenty of room to grow. Also drive it daily when the weather is nice. No issues. Haven't seen a "new production" car hold that reliable yet, without modification, so no point to me to even think about selling, If I did I would have to invest even more money on top of the "new production" make it reliable.. Would still have less money into the car I have now. "822rwhp was on stock motor"
I think the displacement argument can go both ways. Yes, heavier parts within reason, but takes less work to make the same amount of power. Just look at the big inch LS guys. Ran into a guy with a CTS-V making over 700 on E85 with a fuel system and pullies. Everything else was stock.