Kind of looks like you are sacrificing one of the key advantage of a submarine, its stealth! A long range MPA could surely pick up the sail bulge unit sticking out of the water, also wouldn’t it have quite a significant IR signature which again could be picked up at long range.

The art work may be putting you on the wrong foot as the page actually says

“When semi-submerged, SSGT can operate at a fast transit mode, achieved by its gas turbines burning kerosene with air inducted through an extendible mast.”

also

When operating in the fast transit mode, the boat operates as a semi-submersible with
the bulb positioned above the surface. These gas turbines burn kerosene (AVTUR/AVCAT) with air
inducted through an extendable mast. The hull form selected minimises surface wave making in
this mode without compromising submerged performance.

http://media.bmt.org/bmt_media/resources/38/SsgtBrochure.pdf

That sounds much like advanced snorkling….?