I didn't run the numbers, but it seems plausible the difference would be that big.
According to Wikipedia[1] a 7.5 quake releases 11 petajoules of energy, while a 7.8 quake release 32 petajoules.
[1] http://en.wikipedia.org/wiki/Richter_magnitude_scale#Example...
Edit: The difference between a 8.6 one and a 8.7 one is huge. The difference is as big as the difference between 0.0 and 8.35, in megatons at least.
I experienced this first hand here in Christchurch, NZ. We had a 7.1M quake and then a 6.3M aftershock 6 months later. Despite being almost an entire magnitude smaller the 6.3M aftershock was 10x more destructive. This was primarily because the epicenter of the 6.3M quake was 40km closer to the city and some unusually high verticle accelerations caused by the strike slip motion of the fault and the local geography.
http://en.wikipedia.org/wiki/2010_Canterbury_earthquake
http://en.wikipedia.org/wiki/2011_Christchurch_earthquake
The Mercalli Intensity Scale was developed to give a better indicator of the actual effect to people on the ground.
Magnitude of Earthquake M = log I / S
(where I is the intensity of the earthquake taken 100km from epicenter, S is the intensity of a standard earthquake)
First earthquake 7.8 = log I1 / S
Second earthquake 7.1 = log I2 / S
If we solve this for I1/I2, we get 10 ^ 1.10 ~ 12.5
> The intensity of first earthquake was 12.5 times as intense as the later earthquake.