Jan,
the problem with softsynths having latency is NOT because of the software itself, it's 75% soundcard and drivers and 25% CPU speed, RAM, and everthing else related to the speed of the computer.

running softsynths on a tablet might be on the edge of being available with win8, but I'm not sure it's even ready with that yet. you'd need a good USB soundcard with low latency drivers available for the windows tablet and I'm not sure you'll find that yet, although they may show up soon after win8 is released.

most of the latency is going to be due to the soundcard and its drivers. the rest of it is due to the speed of the CPU and motherboard resources, which, I'm not sure a tablet is powerful enough yet to handle very low latencies.

you might could get close IF you can get an external soundcard that could work with the tablet, but then you're back to using an external device running your battery down anyway, so the ketron may be your best choice at this time.