I encourage your giving it a try, but just for the sake of fun, let me make a couple predictions (based on that I was field-testing something similar to that before the advent of the BigShot)
First, tennis balls are light and require no real 'aim', just pull the sucker back and let 'er rip. With shot bags, your aim (unless it's at toss bag heights or less, at which point you don't need a slingshot) has to be dead on.
When you pull that baby back you may get a little shaky from the tension and it's hard not to have your face directly in line with the shot (use at LEAST safety glasses, better, a helmet w/ face protective screen.)
#3 a real BigShot has a pouch with a tug strap directly beneath the pouch. The shape of the pouch keeps the shotbag from shifting or falling out upon release. Yours it flat strap shaped, not a recessed pouch and your bags will be falling out the side upn release.
Fourth, The furthest you can pull is the distance from hand to hand, a la bow and arrow. This will limit the distance you can fire. A BigShot, on a pole you can pull back twice that distance for shots in the 25 - 30 meter range.
10 oz bags may work for you, but 12 and 14's you get a lot more 'recoil' which, with a pole-mounted BigShot you hardly feel.
Let us know how it goes.