Hi jmac2000,
Although Energy might list the sensitivity of the RC10 at 91dB - in all actuality, they are not even close to this. There are many ways manufacturers fudge these numbers. The proper way to measure / publish sensitivity is by listing the speaker's sensitivity as measured anechoically.
As measured at the NRC, the Energy RC10's anechoic sensitivity is 85.5dB. The Sierra-2's sensitivity is 86dB (anechoic). If you are OK with the output levels of your RC10 with your current amp, you will be fine with the Sierra-2 as well, (Sierra-2 actually have slightly higher sensitivity)
Here is a link to the RC10's true sensitivity measurements:
http://www.soundstagenetwork.com/mea.../energy_rc_10/
Hope this helps!