I was just worried because I've seen the video to upgrade to the sideshot mag and they had to remove the rubber balls that retain the OEM FX magazine in position.
Check out the 10:50 mark on this video when Giles removes the rubber balls.
You should be able to use each magazine without removing the balls. I didn't remove my rubber balls! You'll have to try it and see if it works out for your Impact.
It has to fit since the guys shooting Impacts at Speed Silo EBR both Sportsman and Pro were limited to 18 shot capacity factory magazines, if they were speed loading mags and not single loading. I’m pretty sure the 3rd (4th) place finisher in Pro shot an Impact.