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i have 3/8 at 650 hp, with a carter mechanical race pump. my pressure gauge hardly drops at all during a run, so i couldn't tell you how close i am to the limit w/my combo. put a fuel pressure gauge in it. then you know.

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The only true test is watching the gauge throughout a run. If it will hold the same PSI start to finish, you are good to go.
On fuel line size , so many variables come in to play, it is very difficult to put a hp number to any particular fuel line size.
Length of the line, number of bends, G force, pump pressure
and any variance in voltage will change the rate of flow. So the real answer is "it depends".
The end result needs to be will there be enough fuel delivery to keep the bowls at the same level at all times. That is a lot easier with four needle and seats on a tunnelram/dual four combo. A high hp dominater is one of the toughest to feed. Only two needle and seats, and often lots of hp.