You should be fine. Your receiver can handle the surrounds' impedance just fine? Surround effects are very minimal for most movies, and in some movies it's pretty much non-existent. The levels they play are typically lower, and surround speakers are typically closer to the listener, relative to the front three. That all said, Audyssey cannot do anything for a lack of power.
Just level match (which Audyssey will do for you anyhow), and enjoy. I would base my Yamaha decision on cost/value. If only a bit more, go for the bigger model, if a lot more, then don't, pretty simple. If you use anything close to the max capability of either of those amps on those speakers, you won't be able to hear a thing anymore, totally deaf, and all of this talk will just be for, well, naught.
In my multi row theater, I do have an amp powering all 7 channels, but I would be perfectly confident in driving the four surrounds with the receiver. No problem at all, and they don't have 98db sens, not even close, are you kidding me.
