Problem with individual bolt on mods is they all net marginal gains. Lets say something like a performance intake gains 3% hp and your rig makes 140hp, well congrats on the 5hp you'll never really feel.

For example, my buddy's 04 cobra was running below optimal numbers on the dyno during running. They swapped the stock,intake for something more appropriate and he gained over 50hp, but his supercharger moves a ton of air. Hence a 3% gain on a 680hp motor is significant. Do three bolt ons and you,can gain 100hp then.

All the bolt ons I've tried never did anything for me. Even a re gear just was a mild improvement. The starquest turbo swap, now that was a world of a difference. Doubled my power.

So Dohc heads won't do anything. Like go on Edmunds and see how the late model montero 3.8 (I think) went from single to dual cam in a year and it only lists a few hp gain. There's just nothing more to flow through those extra valves. Now if you did the whole routine with stealth turbo motor, well that's over 300hp waiting for you. (And something I'm debating about for my next rig between a 2.4 dsm setup)

Also, dorky mods tend to do more harm than good. Cold air intakes on 4.6 Ford's can absorb heat causing the air temp sensor to get hot and give false readings. Depe,ding on a setup, a bigger throttle body might cause a slight power gain off throttle as it let's in more air than the ecu expects for that possition creating a lean fuel mix, aka not good. But at a more open throttle it won't offer any gains as a n/a won't realy have any way to take in more airthan it needs. Ebay ecu flashes and chips also just make for a leaner fuel mix. Hence you run less gas and gain hp, at the expense of engine longativity. If a dude with a laptop in Russia can write a ecu flash and workers in China design a cold air intake that produce real results, multi billion auto corporations would do it from the factory themselves.