What's the difference between using the 29W, 61W or 87W for fast charging an iPhone 8 Plus?

No, because the output is immaterial. The phone will only draw the amperage it allows by the internal circuitry, regardless of what the adapter can put out. As the phone nears completion, the draw reduces. A 87 watt adapter will charge no faster than the 29 watt adapter.

Think about your home circuitry. Most outlets are 20 amps. But if your TV only needs a couple amps, that is all it will draw. If something goes wrong and the full 20 amps flow through it's circuitry, the TV is fried. Same for your phone.

Here's information from Gizmodo where they did several tests:

https://gizmodo.com/we-tested-iphone-fast-charging-and-you-should-definitel-1820 483924