HP depends on what cams, head flow, etc. matching that compression, but if you just want a ballpark number I will give you one. Assuming a purpose-built engine, figure the 5.4 is 50 cubic inches bigger than a 4.6, so using the same basic compression/cams/heads with this intake figure it will be about 40-50hp and 40-50 ft-lb torque more than a 4.6 DOHC built with same specs. The 5.4 has an enormous 4.165" stroke and can build more bottom end torque therefore making it more streetable than a 4.6 at any given HP level in naturally aspirated form.
The stock Navigator engine produced 300hp @ 5000rpm and 355 ft-lb @2750rpm. It came at 9 1/2 to 1 compression and low duration cams (5000 pound SUV meant to pull rich people's boats and stuff). If you put this intake on a stock Navigator long block, that is the same 300HP you will get. You can get Probe 5.4 pin height forged flat tops for cheap at Summit and have over 11 1/2 to 1 compression, stick in 96-98 Cobra cams, this intake and have a nice torquey engine with 350hp for cheap. Personally, I prefer the 03/04 Cobra heads over the Navigator heads, so I would start from scratch instead of looking for a Navigator engine (plenty of 5.4 sohc blocks out there).