Yes it is small - normally R7 260X is 1/3 of R9 280X. I try it for a lot of algorithms X11, X13, Eterum and gives around 1/3. Only for quark gives a little bit more than 1/3 of R9 280X
I am trying to compare the characteristics of some cards, but do not understand what is crucial. Core frequency? At the moment I only understood that the frequency of memory and amount of memory are not very important.
Core frequency and FP64 is very important. As you can see on this
http://www.anandtech.com/show/9387/amd-radeon-300-series/2R9 380 is similar to R9 280 except it has weaker FP64. R9 380 has 1/16FP64 and R9 280 has 1/4FP64.
R9 380 is obviously slower for mining XPM but it is good for ETH.