You will see your biggest gains with your initial mods. The gains thereafter will be much smaller. From what I've heard, if LTH was your initial mod, you would probably see around 30 whp gains from just that.

Horsepower is relative to altitude. The closer you are to sea level the more horsepower you'll make. Of course the Z will make the best horsepower at sea level under low humidity and cooler temperatures. So if you want to see what it's capable of making, pick a cool, low humidity day to put it on the dyno with properly corrected SAE.

To say it will make a specific horsepower/torque level given full bolts on's is; a wag at best. Given the right climatic conditions, dyno used, altitude, parts used and correction factor who to say, it's all relative.