In my opinion you will want HLCD type speakers in the tower, such as wetsounds, bullet, nvs etc. I believe you can put the 7.7 Bullets in the factory MC cans, but I have only heard this, not done it myself. Two is plenty for me and I can hear them crystal clear while riding. The key to getting them to perform properly is to make sure they have adequate power, i.e. 150w RMS to each, with say the new HD JL amp. A friend has the same two speakers with a different amp and I can't hear his while riding.
Pay attention to max vs. RMS when looking at how much power you have going to each. In my opinion, if you upgrade the tower speakers to some good HLCD speakers with good power you will not have a need to add the speakers to the rear kick panel--the tower speakers will be more than enough and will fill the boat.
The JL's in the boat are great speakers so I would leave them, but I have read that they are typically a little under powered from the factory. If you bought a new amp for the tower it might free up some power for the cabin speakers, but again, once the tower speakers are upgraded you will probably find there is plenty of sound in the cabin.
It is important to make sure you are adequately powering the speakers you have before you add more. Otherwise, adding more underpowered speakers is a waste of money and will be counter productive.