If I had money to burn, not as in disposable income, but as in lottery rich, I'd love to give this a go. Stick 10-15k into the engine and see whats possible, but as a normal working class person, I could never justify it, no matter how much i'd love to.
I had my car almost complete, all I needed was some suspension pieces before the inevitable happened and I would begin thinking about gaining power figures. And do you know what I did next? I SOLD the car! For the same money or possibly less than I would have spent in buying the zed, modifying it for the previous 2 years and then finally building the engine, I bought a 500hp V10 which will lose very little money over the next few years compared to the 50%+ I would have lost on building the engine.
My point is - if you're loaded and money gushing from your pockets then by all means, give it a go, I'd love to see figures, but for the normal working class, it is genuinely a massive waste of money. If you need/want more power, sell up and add a couple of grand to the selling price and buy an e46 M3, or add the full £8+ and buy an e60 m5 or something, much better way to spend your cash