I have one. Its a quality piece and works well.
There are two ways to go with the tune, safe and not safe. If you want to squeeze every last hp out of your motor you can tune it with the spray. In other words, you could increase timing, add boost, etc. However, now you're in a situation where your motor is in danger if the spray fails for whatever reason (runs out of water/methanol, nozzle fails, power to the unit fails, etc, etc). My reservoir runs dry all the time because I forget to keep it filled and its pretty small to begin with.
My car is tuned with the spray turned off. Therefore, when the spray comes on, I'm lowering air temps (increasing safety from detonation) but also lowering hp because I'm taking up space in the combustion chamber with something that is not combustible. So, at the end of the day, I'm looking at my Snow unit as a safety device, not a second power adder.
Now, you can always add stuff to your water mix to add combustibility. I believe Snow sells a nitromethane mix. I have never used it and don't know what it does (other than going boom). I use windshield washer fluid (the kind with methanol in it). I don't believe its used for its combustibility but for its additional cooling capabilities over straight water.
Whether you actually need a Snow kit is debatable. You already have an aftercooler. Have you logged your IATs? Do they get higher than you'd like? You mentioned high summer temps. Keep in mind the Snow kit is only activated at a certain boost level or rpm level (preset by you). So, when cruizing around, its not doing squat. Are your temps high in the summer under normal driving conditions? If so, look elsewhere for a solution. If you are looking for some added protection under WOT conditions this may be the ticket for you.
If you don't really need it I would pass on it. You're just adding complexity and taking up space for no reason. Check your IATs first.