The elephant in the room when talking about power is that all the low hanging fruit's been scooped up by the manufacturer already. The same tricks that make power also improve volumetric efficiency and therefore fuel efficiency, they have everything to gain from maximizing that. It doesn't cost a penny more to make the air intake larger, variable cam timing + variable intake runners means they can run a huge wide open head and still deliver good low rpm driveability and emissions, modern manufacturing has even made cast log exhaust manifolds better than they ever have been, throttle-by-wire allows a large throttle body that's not jerky off-idle like the old days.. they didn't really leave much on the table.
So if you compare the torque a 5.7L Hemi makes (410 lb-ft) vs. the highest powered naturally aspirated engine ever put in a production car - 2023 Corvette's 5.5L engine only makes 12% more torque. Adjusted for displacement that's 16%. The 'vette's engine revs higher of course but at RPM's that both engines cover, all else equal the stock Ram Hemi is only 16% behind the most bad-ass NA production engine ever and it takes four camshafts, individually CNC'd heads (not as-cast like volume heads) and all sorts of cutting edge race tech to get it there.
Short of boost or sauce, how much room do you really think there is for DIY improvement?
My other car is a 94 Mustang 5.0 rated from the factory 215 horsepower. There's probably 100 horsepower worth of low hanging fruit sitting there in the heads, intake manifold and camshaft... but that was close to 30 years ago. Nobody leaves that kind of power on the table anymore, there's not 100 horsepower hiding in a Hemi, in fact other than raising the rev limit I doubt there's 35 and most of that is probably hiding in the exhaust manifolds & mufflers.