I agree with what you're saying but me personally I'd always have a fear of any engine that's been taken apart to be rebuilt. That's just me, maybe I don't have enough faith in some mechanics, to make sure they did it "right".
To exceed 450 horse and have reliability in the Zed you need some serious cash. There's not a great deal of room in the Zed for TT's and SC's and this causes potential risks. I'm not saying there are risks, but potential ones. I've seen enough SC Zeds and their stories all different and there's a lot of tinkering needed and additional parts to make the VQ work safely and efficiently.
I want to take the simple route and trust GM built the engine in a controlled environment tried and tested.
A built motor can be used with many differing brands, some fail, some are rated as crap and some are excellent. Again that would worry me. If I piled £15k to forge the engine and throw in a SC, I'd be devistated to find continual problems. There's a lot of additional parts that make up the bigger picture and again I'd fear the worst. I'm not saying it will go wrong, but I'd worry. Especially as I'm not a mechanic.
If I had your ability with knowledge and cars I'd have done exactly the same as its a challenge, my route is the simple way.