It's just a figure, basicially you measure the air temp going in, and going out, then simply find the drop, divide by the temp going in x 100 = % efficiency, gotta filter in the Ambient temp as this will affect temp.
I managed to find the figures for both the original intercooler and the ST one, As with new intercoolers the should provide you with thier efficiency ratings tho most, if any, wont, and half the time they don't do much better, only worth getting tried and tested ones from eg forge motorsport, but the forgemotorsport intercooler for £300 only drops intake temp to 39 degrees C as opposed to 51 degrees,, not a huge drop considering that the temp is somewhere around 125 degrees going in, tho none the less it'san improvement and on a highly tuned lump will give 10-20% more power alone, but this is only if u've spent £kkk's on tuning.