Sorry for the late reply here. That's not the best information, because top speed would occur after the hill, not a launch. And highly unlikely it'll be 40mph- I would expect the launches to be around 25-30mph to get up the hill w/o a chain lift and it might hit 30-35mph after the hill. The banks and length of track wouldn't support 40mph without requiring mummy-like braking. 43mph on Manta is the slowest Mack launch coaster out there- all their others are 62mph or faster.
This will be much more like firechaser express at Dollywood- less the backwards part. which is a 16 and 20mph launch with top speeds of 35mph. But those slow launches are because it has a chain lift which Slinky won't. However; Even going backwards at the end, it's only a 39" height requirement vs the 48" requirement on manta- which is exactly what they need a Slinky
That makes sense. Any faster and those helixes and overbanked turns would be more thrilling than what they are looking for. I am a terrible judge on height but the hills on the coaster appear to be around 40 to 50 feet high. So, the launch speeds you quoted should do the trick. The Backlot Stunt Coaster at Kings Island has a 45 foot drop from the helixes and its max speed is 45 mph. Due to the winding track, they need a little extra oomph to reach the top.