I just googled for 2015 Sub with factory roof rack

your screwed!  I was going to suggest a set of "Q" towers for the front, but there is no way to put them on the way that roof rack is mounted.

ST's suggestion of the suction cup rack is probably going to be your best bet.  a set at the very front of the roof, and a 2x4 or something to go across the factory rack at the little gap you have to work with would at least give you some spread between the front and rear bars.  might even be better to put a second set of the suction cups at the very rear end of the roof.

if that factory rack is mounted into a track?, then I'd pull those rails off and use track mounted system instead - those you can find.

other alternative would be to mount a trailer hitch to the front and use a hitch mounted rack at the front such as the Yakima Drydock  and a two-by at the back- or just go with a trailer.