It's difficult to quantify the efficiency loss here. The P100D is nearly 200kg heavier than the 75D. While I have no evidence to support it, I think this difference would account more than just a handful of kms of efficiency difference. Anecdotally, when my car is seated with 4 bodies (+250kg), my Wh/km goes up from around 220->250. That's a > 10% decrease in efficiency.
I also have 21" performance wheels on mine, which takes out another 5% at best, and up to 10% at worst. Those wheels are very grippy (in a good way).
Then again, when all's said and done, I didn't buy the P model for range reasons...