With a high volume dust collection, you’re already crippling it by using a 6" to 1.5" reduction
The design is for a cross sectional area of 28.27square inches
But by reducing it to 1.5" your cross sectional area is 1.77square inches
This reduces the flow by forcing the blower which is intended to pull volumes of air, into a vaccume which is not the intention of the design of a blower like that. You’re much better off to get or fabricate a dust shoe that has much less of a reduction and you’ll get more flow out of the dust collection and thus more dust extracted at the point of creation.
IF you do add a port in the rear of the enclosure, that will only help the debris that’s airborne, which most of which will land within the enclosure before being sucked into the port anyway…
IF you do add the port, I suggest also getting a 4" ducted dust shoe… This will result in a 12.57square inches. using 4" at both the dust shoe and the rear of the cabinet will result in a total of 25.4 square inches of area to allow nearly the intended flow rate of the dust collector. . .