Hi Herve,
D1 introduce a voltage drop that impact on the whole system efficiency. This loss can be calculated an depends on current and diode performance in terms od forward voltage drop.
I used an ideal diode circuit to replace D1 during my last test at the bench only. I have not introduce it on my final circuit because of the space on the board.
You could find out a lot of these circuits here :
https://www.google.it/search?q=ideal...L&ved=0CCAQsAQ
You can also google " ideal diode solar " for additional info.
I do suggest to calculate the power saved and evaluate if really worth. It is up to you !
I will really start without it.
Best regards,
iw2fvo, Ambrogio