There's a fella on the youtube who made a few videos about the ST that I found interesting. I don't agree with everything he's concluded, but that's okay, we're all different folks and can look at things from different perspectives. He has a video
here that talks about some of the modifications you are thinking about. I'm of the opinion those snorkels are designed to flow air better than having nothing there to funnel or direct the air at all - that doesn't mean they're optimized for the best performance if you start changing air filters and exhaust, but i do believe they will flow air far better than not having them at all. No harm in trying it out if you're curious though.
I put the Delikevic slip-ons on my bike and I really do like them. They are a little louder than stock, but not by too much (I kept the baffles in). To my ear, they sound throaty and nice - but some may disagree. They absolutely will accentuate or amplify the decel popping. It seems to be a common belief that most aftermarket exhausts will do this. While improper install can be a factor, I do believe the decel popping is there with the factory exhaust, but it's muffled more and likely mitigated some by the catalytic converters that will be removed when you replace the stock silencers. Whatever you try, you can always put it back the way it was.