LRF (BBeB) is a binary format, isn't it? Theoretically it should be faster for your Sony Reader to parse than ePub, which is essentially XHTML. Did you find LRF to perform better?
One problem you might be facing in the future is if you want to convert your LRF books back to something else (who knows for how much Sony is going to support LRF). The conversion process isn't always perfect and it may look different from what your original. Make sure you keep your original ePubs in Calibre.