Plogs are a waste (IMO), functionally 1/2 a header. Seeing as the exhaust manifolds are the biggest restriction, plus that UBend, I would suggest headers as a first mod. It will set up not only what you want to do, but anything in the future too.
Good throttle gains, feel, result from TB and SC housing opening... But as mentioned, any change in air intake, exhaust, or sc rpm (3.4) should be addressed in the PCM in order to ACHIEVE the best results from adding/changing those functions.
A Wideband isn't necessarily needed up front and is a big investment, but will give the most accurate reads of that your combustion is doing. But that said, if you plan to drop pulleys, you'll need to monitor for KR in some form. The cheapest, easiest is an Android Phone with an ELM/BT OBDII reader along with the Torque App.
You shouldn't drop thermostat temp unless tuned for it in PCM, again, no gain unless fan on/off settings are changed as well.
Plugs... Eh, @3.4, not sure they're really needed to be cooler. But add 1.9RR's to that mod group and a range cooler would be set for sure.
Keep in mind the benefit/cost relationship...
CHEAP/FAST/RELIABLE
You can only have two of them at any time, chose wisely. Read the modding your 3800 thread, read it again, then some more.
But yeah, with just a few simple, easy, safe mods, you can give your GP a little punch. A word of caution though, it'll leave ya wanting more. Then a little more, lol...
