Adding an 8 or 10mm SS extension, won't add firing preload as such, but it will limit the amount of swept volume, and the piston forward travel, in essence leaving the spring unable to reach full expansion so the spring will still be under some compression at the end of the cycle. This may prevent the bounce you think you're experiencing to some degree.
The downside of reducing piston travel / swept volume, is that it will likely reduce power. In your situation, it may not be a bad thing.
I'd also recommend chronoing with JSB Express 7.9 as well as the 8.44
With the SS extension fitted, you will notice the barrel drop further on breaking. The extension leaves the piston further away from the cocking shoe.