I went from an ST1300 to a 919 two years ago. The ST was a great mile muncher but, like you, I grew bored with it and wanted more spirit in my steed.
You give up virtually all protection, unless you add a windscreen. At freeway speeds this can get tiring, especially if you are used to the protection of a sport tourer. I don't ride below 40 degrees so I can't comment on 25-35 degree riding. But a messenger bag did the trick for commuting with a laptop and some gym clothes. Rain is no problem if you have the right gear.
From my perspective, you may miss the shaft drive. After I started doing chain maintenance I missed the forget-about-it shaft. You may miss the protection from the weather. You will miss the panniers but luggage options are available for the 919. You may miss the comfort that the ST11 has on long rides since they are such a great mile muncher.
In return what you get with the 919 is a spirited bike that is a hoot to ride. It will feel like a toy in weight compared to the ST1100. You won't miss having to shed plastic to do maintenance. You'll have much more fun on the road and go out of your way looking for twisty roads. You'll love the powerband and the sound of the 919's I4. You'll also love the people of the forum you have found here at WT and the vast knowledge on 919's here.
If you have the chance, find a 919 and take it for a ride. All the feedback in the world won't replace your own test ride. Good luck with your decision and keep us posted. And stick around! Although there are many 919 owners here, there are many other bikes here.
BTW, I moved on from a 919 to a new Bandit 1250 last year. Not because of any problem or dislike of the 919, but just because I fell in love with the new Bandit