Space and weight of batteries is always an issue. Toyota has moved the inverter to the front of the car and there is only the two batteries left in the trunk. Potentially they could move the 12V to under the hood, but it sure looks crowded in there now... This would allow them to use a normal vented battery worth $50 or so, instead of the $300 sealed AGM unit.
LiIon batteries would save some space and weight, or increase capacity while maintaining space the same. However, I suspect the battery or the drive motor is not the issue. The motor is rated at about 70 KW, or about 94 HP. However it only is used for about 45 HP. The limitation is probably current flow through the inverter. To increase electric power would require a major upgrade of the inverter, and make it bigger. Where would it go? Back to the trunk?
The other really big issue is life of the LiIon batteries. Toyota had gained an excellent reputation for reliability with the NiMH batteries. They are better suited to the hybrid service when it comes to battery life. How would they handle the issue of the batteries not lasting the normal life of the car, and that the LiIon batteries are much more expensive. Hyundai has offered a lifetime warranty, but time will tell if that is going to become a financial burden. I wouldn't touch LiIon unless there was an ironclad warranty.
With respect to more speed in EV mode, my guess it is not a battery or power issue. It is the maximum speed rating of MG-1. With the 2012 model year maximum speed was increased to 13,000 RPM. I'm thinking they are not likely to make another major transaxle change just one year later. Play around with this Prius simulator
and you will see how maximum speed in this version of the Prius is limited to about 42 mph. MG-1 in this case is limited to 6,500 rpm. Above that the ICE has to start and spin.
I may be pleasantly surprised, but I suspect they are simply selling as many 2012's as they can, as they have nothing new to offer for 2013. My big hope is that they would finally fix the trunk lid so you don't bang your head on it.