The power mode simply changes your shift points. The engine revs a little higher allowing more line pressure to build giving a little crisper shifting...or something like that.
From all I have read the crisper shifting is actually better for your tranny since the clutches slip less.
I have 140k on my VX...power botton on 95% of the time, no issues, burn no oil and still get 16-20the mpg with large heavy offroad tires/rims and a lift.