FWIW I have been riding this setup for about three months now. Absolutely no pedal scraping going on. It affords a lot of extra agility with no negative consequence. And I can't barspin because of the angle of my stem, and anyways because of the rake on the fork wouldn't be able to get the wheel past the downtube anyways, but that's fine, because it's very comfortable (and I wasn't doing this for barspinning anyways). I notice no excess twitchiness and no issues riding hands free. My other bike, with a 700 in front, feels like a boat when I ride it now. I really like the 650 in front.
So, thanks for the people who have tried this and encouraged me, it is a fun, fast, comfortable bike and I'm glad I went for it.