Hard drive cooling is definitely not up there when it comes to performance increases.... just have to keep it from burning up really.... unless your case has near to no ventilation and you've got some raptors running at 10k rpm.... otherwise... 120mm or 2x80mms blowing intake onto the HDD bays is more than you need (hell... if you had good ambient case temps because of good airflow management[like oh...I don't know... stock antec 900 fans?] the need for these fans wouldn't be present and you'd spare yourself from unneeded decibels)
As for extra cooling through the PCI slots:
#1: the exhaust fan should probably do a good enough of a job
#2: If you're willing to cover up a PCI slot for cooling... why bother with a measly 9.31CFM and a measly 1 PCI slot? Do what I did with the old computer I gave to my GF! BAM! 30ish CFM intake! Also could tilt it more and add paper siding (to tunnel the air better and to allow the use of one of the PCI slots)....