Considering the new antec cases, which is very similar (if not identical interior-wise) to the sx1030/40s come with a blowhole on the side, (next to the handle)...it probably wouldn't disrupt airflow that much. Also, the alienware (antec) cases have a blowhole in a similar location, so it should provide some benefit in terms of card cooling.
As far as the size goes, i think the oem and alienware use 80mm fans for their blowhole.