With a snug neoprene neck gasket (punch through, non adjustable collar) you're going to take on some water in a swim, but not that much as long as the gasket is snug fitting. I used several different semi-drytops with neoprene neck gasket for the last 8 plus years and they are (in my opinion) much more comfortable than a latex neck gasket.
As for how much water - not that much, but it depends on how much you swim, how long you swim, and what type of water you're swimming through. I used the tops mentioned above for kayaking and the neck gasket would take on water down the back of the neck when doing rolls, and take on water through the front of the gasket when punching through waves and holes where I'd take a hard shot of water to the front.
When you flex your neck muscles the neoprene gasket doesn't retain as tight of a seal as does the latex gasket, allowing in the water.
After several hours of kayaking and many rolls, the chest area and back would be wet at least mid way down my chest/back, sometimes further. During summer boating this wasn't an issue, but during spring and fall boating in cold water and cold weather, and/or for long stretches this did not help me stay warm and comfortable.
I upgraded to a latex neck gasket drytop this past spring and while I still prefer the comfort of a neoprene gasket, I can deal with the latex gasket (neck burn/chaffing) in order to stay dry/warm during early spring/late fall runs.
In the end - depending on when, where, and what you are rafting the SuperNova may be just fine for what you do. I wouldn't hesitate to wear one on most of the rafting trips I do.