As far as I can tell it takes about 200 miles to fully break in the engine. After 200 miles and an oil change you should see peak power from your engine. You should continue to see this power after each oil change up until you have at least a couple of thousand or more miles. I know by 7,000 there is a bit of a drop and it continues up to at least 15,000 to drop a little after each of the longer races.
The Zonda-R fully broken in with the High RPM Turbo and fresh oil should top out at 956 HP I think.
btw do not bother rebuilding an engine until you have several thousand miles on it, Especially on cars like this. Even if you have dropped 5 or 10 HP it makes very little difference in the performance of a car like this and is very costly. Better to wait unless it has gotten to the point that you can no longer win races with it.
Edit: I have tested this btw with several cars I have saw HP increase on all that have over 100hp during the first 200 miles and no increase afterwards until the engine starts to wear down and needs rebuilt at which point I would imagion that you are back to breaking it in again. With that in mind if your car while new and after an oil change has say 750hp then later after breakign it in has 775hp do not bother to rebuild the engine until it drops below 750 after an oil change else you are just throwing your money away.
Also the change in the OP is more than likely due to an oil change.