@Garota: Except it doesn't even do the same thing as the obis. It enhances the bonus itself, while the obis just make the bonus apply 100% of the time. You'd wanna use this WITH the obis, or else it'd be a lot less useful. :P
From personal testing, this does in fact work with cure spells. Just as Dekka has said, it is a 5% bonus.
Bonus stacks ON TOP of the full 35% bonus you can get from day/double weather for a total bonus of 40%.
This cape is best used in conjunction with obi's to fully reap the benefits as with the cape alone, you won't always gain the full day/weather benefit.
Tested in Temenos lobby on lightsday.
Also @ Ragni. The 15% you were experiencing was most likely just the 10% day bonus, leaving you with 5% bonus Dekka and I have both come up with. The only real way to test this is with the correct obi as to not have any day bonus variable.