If you have any desire/time to learn how to tune, i would buy a tuner. A good tune can drop close to a second off of your 1\4 mile. If i remember correctly someone claimed to have run a 99 stock GTP in to the high 13's with a really good tune.
Your decision should be based on your mechanical expertise and goals.
If you have a good deal of knowledge/skills i would go for the headers. Not too hard to install but problems with sealing could call for substantial welding. (I'm not sure about the track record of SSAC headers). Breaking a bolt in your engine block = no bueno, would you know what to do? You allready have a dp which would become useless if you get headers (some types anyway).
Personally i would go with choice FOUR. You get the tuned PCM which you defiantly need (ZZP is garbage). You get the P&P, better S/C and TB efficiency, less KR and more whine! And you buy a Plog, the rear manifold isn't too restrictive however the front is, the plog will help allot. You don't need to lift your car to do this work and if you exchange your cores the price is amazing. All work is done on top and any problems you encounter would be easier to fix.
Choice four would run you ~ $540 and ~$440 after you return the blower and tb cores. If you get the plog used you can knock another $40 off of that price at least, bringing you down to $400. You can probably even get something for the front exhaust manifold if you decide to sell it.
