Those speakers are nominally rated at 200 Watt RMS so they would benefit from the additional power. Though I'm a bit suspicious about Onkyo's claims for doubling power based on their history of overstating rated amp power in their receivers.
I was taking a look at my brother's TX-SR875 when I was in LA this weekend and it runs very hot. So right away forget about the amp's rated output "140W minimum continuous power per channe, 8 ohm loads, 2 channel driven at 20-20khz" claimed in their specs. Also with one single transformer in that receiver, what they claim is biamping or bridging is more likely just biwiring.
Anyway you won't hurt the M50s and you never know there may actually be an improvement in sound so its worth a try.
