I don't know of any proper test of the Yamaha, but it may well be pretty good (and not mid-fi) if it compares to e.g. the Yamaha AS700 tested here: http://i.nextmedia.com.au/Assets/Yamaha_AS-700_Amplifier_Review_LoRes.pdf
The need for power largely depends on the music you play, the size of the room and the sensitivity of the speakers (88dB in this case). Unless the room is very large 150 watt per channel should be OK, even if (up to a point) more is always better (for cleaner response to dynamic peaks). More amplifier power does not need to cost much, of course. Better speakers would make a bigger difference, but will cost significantly more.
The need for power largely depends on the music you play, the size of the room and the sensitivity of the speakers (88dB in this case). Unless the room is very large 150 watt per channel should be OK, even if (up to a point) more is always better (for cleaner response to dynamic peaks). More amplifier power does not need to cost much, of course. Better speakers would make a bigger difference, but will cost significantly more.