I've never fully understood the technical reason for the hiss, but all our cars seem to have it HFCs at least. It happens on decel at a certain RPM range. Personally I kind of like the sound, but I'm wondering if it will change or go away with LTHs. I'll find out soon
Re: oil temps, keep in mind the ambient temp is warming up everywhere as we enter summer, and this has a huge impact on your oil temps, it's likely the reason you're seeing a rise. I'd say as long as you're not regularly staying up past 240 (e.g. 260+) you don't really have anything to worry about. A good oil cooler is the solution if you're tracking the car, or driving it hard in hot climate. The exhaust setup isn't really going to change the oil temps situation much, except that maybe you're more prone to pushing the car harder now that you enjoy the exhaust note more.