I know I'm late to this thread, but I've been using LiFePo batteries for a couple years now with mixed results. I can't comment on the Ballistic specifically, but the technology is the same.
I've had 8 cell MOTY batteries in my VFR and RC51. The VFR is fine for 8-9 months out of the year. Cold morning starts are tough, usually with the clock and trip meters resetting due to low power. My friend with another VFR had the same issues and was able to trade it in for a 12 cell. He reports no issues at all with a 12 cell on his VFR800. I haven't used my VFR for commuting, so it didn't need to start in the coldest part of the day, or sit outside at work overnight, and I've been ok lately with the 8 cell.
On the RC51, the 8 cell simply isn't enough. Even on warm weather starts, with an overnight (or longer) shut down, it wouldn't start. Just not enough CCA to get the big VTwin cranking.
The 8 cell are really better suited to Inline 4 sportbikes. Small displacement, small pistions, short cranks, etc., all easier to get spinning with a cool engine.
I'd go with a 12 cell for any V engine, IMHO. Even the 800 V4 needs the few extra CCA's over a 600 or even 1000 inline. If you live in a particularly cool climate, do not even think about an 8 cell! I live in GA, and our mild winters are tough on the 8 cell starting.
Once the engine warms up, however, they work just fine. I would use another battery to boost the RC51 starting, and never think about it again all day riding.