So I've been soaking in the Sinnoh mons that dropped, and yeah, the Sinnoh starter spawn rate appears to be really high. Chimchar was the first one I caught, yet on my first day hunting Sinnoh mons, I've managed to find so many Turtwig that I already ended up with a Grotle at the end of this day. Today, I went out again and found an increase in the spawn rate of Piplup and a few more Chimchar, allowing me to evolve mine (they're all good IV). At this point I now have a Grotle, Monferno, Prinplup, Staravia, Bidoof and Kricketot.
Still haven't found a Buneary yet (well, I did, but it ran away) but I'm glad I also found a good IV Larvitar today as well...