There's been some solid research by someone with the USGS that reveals the New Madrid quakes weren't quite as large as first thought; got no media pub at all because it wasn't scary and exciting.
They were VERY powerful earthquakes, and would be extremely destructive if they occured today, but they are no longer considered the largest lower 48 quakes known; Tejon Pass in the 1800s in California, and the 1906 San Francisco quakes were more powerful.
Also, the reccurence time on New Madrid quakes of the size of the ones in the early 19th century seems to be very long; 400-1000 years.
The powerful earthquake threat in California, Oregon/Washington, and Utah is still FAR greater than that around New Madrid.
Unless the interval period was updated as a part of that study, the intervals are around 200-220 years with the New Madrid fault.
You may be technicly correct with your comments regarding specific strengths with these quakes, however, letters circa the period when the New Madrid fault last broke, do show significant shaking as far away as Boston, hence the bell ringing. With this in mind, I believe it is reasonable to consider wide-spread damage should the New Madrid break loose.
Because of the increased populace, the relative earthquake standards employed for new buildings, I'd still fear more from a New Madrid quake. But if your new references to 800-1000 years frequencies are correct, that would put things in a new perspective.