If you look in Marks thread you will find a comparison between a stock and a tuned zed with no mods. There were gains in the right places, better throttle control and I belive better economy.
Dynos are close but it depends on operators and what gear the car is in as well as variables like if the dyno has been calibrated well. Stock zeds on the dyno tend to come in close to the given BHP, some have come in low but that can also be down to compression, sparks etc etc. Main thing is to find some one you trust (as well as others) get a dyno reading and continue on that same dyno for the rest of the work.
I think you could be right on the cost effective route.. If I started again, the first thing on the car would be a charger or some mild but upgradeable TT... But I have roughly a 40bhp (and shed loads more torque) increase over when I had the car 1st dynoed (think it was about 228 at the wheel and is now 271 at the hub) and the car performs soo much better so am happy with what I got... For now!!
+1 If I started again my first purchase would be a Vortech