I highly recommend visiting Yokohama. There's a lot to see there. Go to a Baystars game if they are in town. You might want to consider spending the night of the 5th in Yokohama, it's on the way to Kyoto and you won't have to double back to Tokyo. I don't think you need 6 nights in the Kyoto/Osaka/Hiroshima region. Consider making Hiroshima/Miyajima an overnight trip also.
Also, Japan has a great and very easy luggage transfer service called Ta Q Bin (Yamato Transport) . Very easy to use. When you check out of your hotel (assuming it's a proper hotel) you tell the Concierge where you want your luggage shipped and they take out the forms and fill them in and measure your bag and then you make payment to the hotel (like $15-$20 for a normal sized bag). Your bag arrives at your hotel the following evening. You can then walk to and from the train station if your hotels are close and take only a day bag with you. This is really convenient if you have an overnight stop somewhere (like Yokohama, Hiroshima, etc) because you can hop and off the trains easily for an overnight trip without carrying large luggage. Or I imagine you could just have large bags shipped from one Kyoto Hotel to Tarawaya if you wanted to overnight in Hiroshima in between.