Ok did a proper test this time with all the same settings.
And it clocked in at 4hrs 22Mins.
So in summary so far:
All using the same SuperCard MiniSD and the same battery leaving SuperMarioKart
Full Brightness and max vol through headphones:
UnFLashed DS
Orginal Cart: 5hrs 10Mins
SuperKey in DS Slot: 4hrs 15Mins
SuperKey Removed: 4hrs 22Mins
FLASHED DS
DS Slot Empty: 4hrs 20 Mins
DS Slot with Cart: 4hrs 24 Mins
Using SuperCard Lite: 4hrs 24 Mins
So I conclude that having a flashed ds or superKey makes bugger all difference to battery life during normal play...for my DS.
I'm quite sure it makes a difference to the sleep mode though.
Not sure how I'm going to test that unless the difference is significantly different.....
KK
edit - Not as controlled as I would like, but I guess I could put the DS in sleep mode over night and then see how long the battery lasts coming back to the game in the morning.