I easily got approx 25 HP with those kind of mods when I got the UpRev (Milltek full exhaust, Motordyne plenum spacer, HKS Panel filter, Berk HFCs) and could have done a bit more with say a Velocity stack or an 06 airbox on my 04 276 Zed! I think you're basing it too much on your 297 BHP model, I think the 276 model yields better gains TBH. (I am basing approx 300 @ the fly by the fact I get 256.7 at the wheels, at around 15% driveterain loss that sounds about right to me)
very rarely has any of the 276 models cleared the 300ps mark with ease. The mods you have are all very good mods and expensive. The total bill will be much more than the 1k this owner was thinking about.
The 25hp difference doesnt make the car feel alot faster, maybe a bit more responsive and to the butt dyno it may feel alot faster. Ive trawled all the dyno threads and rarely seen 300hp on the 276 without spending more money that its worth i'm afraid.
Remember every dyno reads differently so the actual accuracy is a thing to remember also.
If you want a really fast car dont buy a zed, simples.