Well, pretty much 99% of people who buy expensive stuff have never tried cheap stuff OR have had a bad experience. So I understand the reasoning from both sides. I think there are more tolerance spreads with cheaper stuff. So sometimes the odd mod will be required. Minimum effort though.
If a manifold cracks, you just weld it. I braced mine and modified it to suit a vband wastegate. Then ported it best I could. I also rotated the wastegate outlet so I have the wastegate in the top side by the turbo. Machined the flange flat and it mated fine. Any basic niggles so easy to fix even in the off chance any thing does arise. But these mods aren't always essential or required, I just wanted to do it lol.
I see all these twin turbo peeps running 264 cams and expensive parts to hit 600hp. I've a little NA-t and running STOCK cams hitting 600hp so it can't be that bad. No boost control issues. Another funny thing is my old T61 turbo, XS power ebay jobbie, been running fine (new owner at 400hp also) for NINE YEARS! Not related but mental I thought.
Invest in the ecu and wastegate without a doubt. But manifold, down/midpipes, I've not seen anyone gain from spending out on that. I probably could have got away with a cheaper intercooler too.
For much higher power it's perhaps better to have thicker sections for heat soak issues but where that limit is, I've no idea. It's definitely not at 600hp for a road car anyway.