Each to their own, but personally I couldn't have an n/a Zed, I just don't think they are fast enough in standard form. I was looking at getting one for a few years, but every time I drove one I was just disappointed with its lack of grunt. But once I drove the FI zed it was a different kettle of fish and I had to have it.
Your almost certainly right that your car didn't need 420whp to make it better and you said yourself that you got paranoid about noises and potential problems probably due to knowing that you were pushing the safe limit on stock internals. Is this not what sucked the fun out of driving it?
Mine is only running 400bhp (fly) and is sssooooo much fun to drive. I feel being built and tuned at Abbey and not being pushed to the limit gives it the best chance of surviving and stops me worrying too much.
I'm sure one day in the near future I might start chasing a few more HP as it's very hard not to but I do believe 400 is about right for these cars for drive-ability vs performance.