I have a tank bag - and unfortunately I can't tell you how well it works with the stock tank. Mine is held on by magnets in the 4 flaps and it sticks tight. It would cover the speedometer and all the indicator lights, and you have to move it to the side when you fuel up.
You really need to just ride your bike to several shops, and ask them to let you take the bag outside and try it on your bike. I did that at 4 shops before I found one that worked well on my tank. Mine is a medium sized one and will just barely hold a thin rain suit. It is great for a drink bottle, sunglasses, cell phone, sandwich....and it has a clear map pocket on top. The map pocket window will only show about 40 miles of roadway.....so you have to stop and change it often. My Ipone can be used through the clear cover and my bare finger can scroll though the functions while riding......but I wear gloves and that doesn't work. I have used my cell phone to navigate around a subdivision to find the right street and house - but never at speed on the highway.