99 GTP: Benefits of upgrades

[DrRadar]

I'm doing extensive maintenance on my 99 GTP with 198k miles. Since I'm doing all this work, it would be nice to squeeze some more performance out without significantly decreasing life expectancy. I don't mind putting more money into the car if there's performance improvement. Specifically, I want more acceleration from a stop but don't need top-end performance. I'll be driving mainly in town, and it would be nice to leave those Mustangs and Hondas way behind from the stop light. I'm not racing or running quarter miles.

I've spent all day researching upgrades but it seems that some judgment (gambling?) is needed. I'm not yet certain how much more performance I'll get with the upgrades.

I've become concerned with the damage that can be done by knock retard, whatever that is. Don't want to put the engine at great risk of severe damage.

Based on my reading, these are the upgrades that should be considered in order and what they can do. Please correct me if I'm wrong. I'm already planning on putting in a 180 deg thermostat and NGK TR6 plugs. I'm considering #1 - #5; anything after that probably false below the return-on-investment threshold. The return-on-investment (as well as risk to engine damage) are the decision criteria. Any feedback from those who have upgraded their GTPs is greatly appreciated.

1. Improve exhaust flow: At least U-bend delete. Better is downpipe (w/ cat) and cat-back exhaust. How much performance with either of these make?
2. Smaller supercharger pulley: Probably 3.4" for me. Claims are this will be a noticeable improvement in performance, but here's where the KR issue starts - right?
3. Throttle body spacer: I'm not sure how this works...coolant flow through TB heats intake air? and this eliminates the coolant flow? I'm in Texas, so not too worried about losing heat in winter.
4. Cold air intake: haven't research much, but idea is to reduce intake air temperature. Is this improvement noticeable?
5. PCM: More aggressive trans shifts and more. I put a ZZP in my 98 GTP last year (w/o other mods). Sounds like DHP is highly recommended, with Intense getting some votes. Not everyone likes the ZZP programming.
6. Headers: They would look really cool, but sounds like performance gains are at mid- to high-end, and not sure I would exercise them.
7. Intercooler: More effective air cooling, but probably more than I want to spend. Point of diminishing returns? Any benefits if not racing?
8. Fuel injectors: When are higher pressure injectors needed? Probably more than I need?
9. Transmission cooler (or rebuild?): Don't intend to run high-speed for long periods, so probably not needed for me.
10. Rockers, valve springs: I've read that changing valve springs and rockers (and maybe valves?) can improve performance? Probably more than I want to bite off for change in performance.

[Stealthee]

Skip the Tstat unless you are having overheating problems. There is no proven power gain from a 180 tstat over stock.

1. Get an aftermarket downpipe. Skip the catback. Stock catback is good to over 350 whp.
2. Pulley size depends on tunability. You don't want to drop pulleys without scanning for KR
3. Skip throttle body spacer. It does nothing.
4. Intake doesn't really help much. It just increases sound. The air is super heated when it is compressed by the supercharger.
5. Either buy your own tuner or find someone local who has one and have them tune your car. A canned tune is never going to be better than an in car.
6. Headers are going to help keep away KR. If you get headers you can skip the downpipe from #1
7. IC starts to help once you start going way smaller on the pulley.
8. Stock injectors are good for a decent amount of power. I don't know that number off the top of my head.
9. Tranny cooler is a must. They are relatively cheap and pretty easy to install. They will help the life of your transmission. You can look into a shift kit while you are at it as well.
10. Rockers, lifters and cam is where you will pick up a lot more power.