I am going to due to a major upgrade and this is the list I came up with
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Cold air intake
180* thermostat
3.5" pulley
colder spark plugs
PCM upgrade
Shift kit
Series 40 mufflers
Powerlog front manifold
xp CAM (oil restricted push rods) or 1.9 rockers (springs, retainers)
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What could I be seeing in gains.....
If the list should be modified in any way let me know....

2000 bonneville ssei
136k

Hard to give an exact number. But you will feel the difference. You will be at the point where your tires will become a problem. Time to start considering wider tires.

what size tires do you recommend?

I want to be able to stay up with a 350z

The wider tires will get you grip and traction for off the line. The width of the tires you'll be able to suck in under the fender well will be dependent on if you change your suspension (lower or raise), change your rims (bigger or smaller). A 245-45-17 drag radial will fit without rubbing on most.

Here'* a good thread on tire and wheel size https://www.gmforum.com/t278965/

So the list I have will let me keep up with a 350z....

Also, I posted on another board and I have members telling me I should get headers too, good idea?

Headers is your best choice for exhaust. I went from Ported Manifolds to TOGs and regret waiting so long to go to TOGs

I think it'll will get you on your way to keeping up with the 350z. Its hard to say without hearing some of the specs of the Z you'll be racing, (I mean if thats what ur planning on doing, Legally i hope)LOL The 350z comes stock with upwards of 300 horses. So it depends on what ur opponent has added to his ride as well. Manual transmission, rear wheel drive, turbo, all these factors will come into play. If hes got a basic model, automatic trans, and no forced induction system of any kind then you may be in business. But again like i said its hard to say.

X2....Invest in some TOG'* and open the exhaust up. It'* amazing how much that will improve the giddy up and go!

Headers are not cheap though. Buying them and getting them installed at any shop will most likely cost you at least a grand.

So I am reading this correctly, I should be getting getter gains with the 2nd list
.
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Cold air intake
180* thermostat
3.5" pulley
colder spark plugs
PCM upgrade
Shift kit
Series 40 mufflers
Powerlog front manifold
xp CAM (oil restricted push rods) or 1.9 rockers (springs, retainers)
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Cold air intake
180* thermostat
Headers
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Basically I would like to get over 300hp