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I actually run my setup in clamshell mode while connected to the eGPU and just use one big display. So I'm not a great person to provide a testimonial there. But I have periodically run with the MBP display open and enabled. The biggest difference is a subtle but noticeable improvement in user interface latency, which is interesting given the video signal is going out over an additional cable.

My understanding is that for the 15" MBPs, the built-in discrete GPU would always be used to drive an external display. So the following doesn't apply for them. But the 13" MBPs have the built-in Intel graphics and you can definitely feel a difference driving even one bigger external display using Intel graphics vs something more substantial like an AMD Polaris (RX580/RX590) or Vega cards.