I am interested in getting a bike with 27.5+ wheels (that could features tyres up to 3.25". I'd like to know what is the thinnest tyre I could mount on such a wheel in case I want to have more speed and less "power".
I wouldn't go much narrower than 2.4 on the 27.5+ wheelset which is usually 40 to 50mm wide, maybe 2.2 if it's got 35mm wide rims as some setups prefer. Keep in mind that you drop your bottom bracket quite a bit when you size down, so you might get more pedal strikes and different handling by using smaller tires. I wouldn't go smaller than 2.4" tires on a bike designed for 27.5+ tires.
However, you could likely use a standard 29x2.2 tire on a 29er wheelset on the same bike which should be about the same diameter as your 27.5+ wheel & tire. It's a bit more expensive to have a separate wheelset, but it can give you the versatility to swap between fat and skinny widths.
The usual answer is "Which tire fits safely on which rim?" on this page: http://www.sheldonbrown.com/tire-sizing.html