The February 2011 issue of Motor Trend did a brief comparison of the performance of a 2010 SRT (with 6.1) vs, a 2011 SRT (with 6.4). Here are the results:

0-60 mph- 4.4 sec.- 6.1; 4.6 sec. 6.4
1/4- 13.3 sec.@106.1 mph- 6.1; 13.0 sec.@111.3 mph
Figure 8: 25.7 sec.- 6.1; 25.6 sec.- 6.4
lateral accel: 0.87g- 6.1; 0.89g- 6.4
60-0 braking: 117 ft.- 6.1; 117 ft.- 6.4

Their summation of the 2011 Challenger SRT8:

"This is a monster of a muscle machine that feels more like an oversized sports car. What Ford has managed to do with the Mustang GT- tighten this and button down that- Dodge has masterfully accomplished in the new Challenger SRT8.: