It will not suck more than it pumps out. If you have restrictive media in it will pass less water thus sucking less. The flow from your FX5 should be very strong, able to hit the front glass on a 24" wide tank with ease.
The purpose of the big intake is to decrease the suction so fish doesn't get sucked into the intake. If you must then you can buy the intake kit from Eheim for $20 online.