I made 20 tile long test pipes with input nodes all at 1000°C.
After waiting a minute they saturated to their maximum temperatures. Yes, they never went any higher than that.
heat pipe dragged away from reactors: 1000°C
screenshot
heat pipe dragged towards reactors: 683.33°C
screenshot
Random placed pipe pieces: 800°C
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Expected behavior would be for any placement order to reach 1000°C. Even if it takes some variants longer due to update cycles.