This subject has been discussed before on this forum. See:
http://www.pprune.org/tech-log/28296...mic-limit.html
Vmo/Mmo limits can exist for a variety of reasons. Flutter, Certification limits, Mach Tuck effects, etc. Down low, you can go pretty darn fast without bumping Mach limits. As long as you don't run into a flutter limit or a bird while accelerating, there ought to be enough thrust to get up to at least 500 knots. As long as your controls are still working conventionally, flying into a building shouldn't be hard to do. I've been much faster down low and about the only difference is a much larger turn radius and the greater beating you get from turbulence down low.
If you will remember, both aircraft hit with significant bank angles. This could be the result of an intent to increase multi-floor damage on the part of the bad guys, or simply the result of alignment turns as they got in close.