Wow, a lot of questions in that post :eek2:
I'll try to hit a couple of them..
First of all, Congrats on the Z
If you are going to spend a lot of time at the drags then it would appear you've already absorbed some pretty good info.
Some quality DR's, a hardened ouput shaft and a DTE tranny/diff brace are eecellent mods for the drags.
You can get the brace directly from DTE and the hardened halfshaft can be purchased at a number of vendors (Lingenfelter, 21CMC, ECS, etc). Pricing is pretty close at all these places.
Since you are running close to 600rwhp, then practicing your launches and shift techniques is the next 'mod' that will improve your times the most. You will find (with practice) the most comfortable rpms and clutch release for your own style.
The only other mod you might consider if you're going to the strip a lot would be a shift light. It's not mandatory but it can help improve your shift points while allowing you to concentrate more on the track. I use a Raptor but there are other quality shift lights available. Mine is mounted on the A pillar above the boost and fuel pressure gauges so that the light it is in my peripheral vison but not an interference.
I'm sure others will chime in with a few suggestions.
ps, one last tip that many people overlook is to check your tire pressure between passes. If you have lowered your tire pressure to, let's say 16 lbs, after a pass is made if you check it again it will probably show 18 or 19lbs. The heat from the track automatically reinflates the tire. I have to take air out of my rear tires after every pass to keep it where I want it....