The 5° tilt in the mounting position will not affect the function of the AS GPS HS antenna ...
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You did well by testing the area for interference ...

What may affect the AS GPS HS antenna function (and most do not think to check) is if the big motor wiring harness runs directly under this starboard rear corner ...

Interference in the Heading function (internal compass) may occur when the big motor is running due to the current running thru the wiring harness ...

Now, if interference in the AS GPS HS antenna function (the boat icon orientation) occurs during big motor operation ... It may not be a huge problem because the speed of the boat (using the big motor) will be fast enough to have a consistent "Course direction line" ... (even if the boat icon {Heading function} is skewed during big motor operation) ...

"Course direction" is which way the boat is moving ...(only available in HB above 0.6mph) ...
"Heading direction" is which way the boat is pointing (available at any speed) ...

Rickie