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Can't you do this more efficiently opening several instances of an emulator on a powerful computer?

Not for iOS

You wouldn't need iOS, you could do it perfectly fine in iTunes on Windows or OS X.

1) Setup VM with OSX, 2) Sign-in iTunes account created with a gift card, 3) Download/purchase app in iTunes (OSX), 4) Rate app in iTunes (OSX).

Step 3 might require a physical device, but 1, 2, and 4 do not.