if you're only 6ft away I don't think you need to worry about the sierra's sensitivity rating.
this only deals with watts and speaker distance:
you'll experience up to a 6db loss for being 6ft away from the speakers. every 3 db (considered change in sound level) takes twice as much power as the last. so if your speaker is 90db@1watt/1meter then at 4 watts you get 6db increase, and at 6ft you'll get 90db of perceived spl.
it may take your avr let's say an average of 5 watts to each speaker to reach 90db at 6ft (for each speaker), it would only be about 8-10 watts for each sierra. that's an over-estimation too.
let's say your new speaker is 87db@1watt/1meter, 2 watts for 90db@1watt/1meter, 8 watts would be 90db at 6ft. remember this is not taking into account anything but wattage and speaker distances.
on the downside, i doubt your avr is even 100 watts total in power; about 60 of that is clean output. luckily in your scenario, you don't need too much. i also hear onkyo's are good at leveling sound off and performing compression to make things still sound loud for the listener.
sticking with 90db, it'd be about 20-25 watts maybe for the front 3 speakers (sierras and a 340 center.) i bet someone out there figured out the onkyo's power/distance factors already, and can tell you recommended speaker sensitivities.
now 93 db is a different story and you may hear sound degradation on some level as the amp may compress signals, but you shouldn't be listening to anything that loud for extended periods of time anyhow