I recently used medicham in a game for the first time. Mentally I was trying to figure out what his "attack" power might be, given his Pure Power ability. Based on a cursory reading, I was mentally just doubling his base attack stat (from 60 to 120), which puts him among the more powerful physical attackers in the game - even more so for his mega evolution form (base attack stat of 100 doubles to 200).
I just happened to read the ability again and noted that it actually says that it boosts his attack stat. This leads me to believe that it is therefore not his base attack that is doubled, but rather his actual attack stat (after accounting for IVs EVs, and nature). This is much more favorable for medicham than simply doubling his base attack stat.
Running through the stat formulas for a pokemon with perfect IV, EV, and nature, a level 100 medicham with his base attack stat doubled would have an effective attack stat of:
(((2*120 + 31 + 256/4)*100)/100 + 5)*1.1 = 374
but if you double the entire attack stat, he would have an effective attack stat of:
(((2*60 + 31 + 256/4)*100)/100 + 5)*1.1*2 = 484
A substantial (30% difference). Any idea which of these two scenarios is actually used behind the scenes?
The latter calculation would give him the equivalent attack stat of a perfect pokemon with a base attack stat of 170, giving him one of the most powerful attack stats in the game. His mega evolved version would have the equivalent attack stat of a pokemon with a 250 base attack stat.