I have just finished a kayak with all the hull and half the deck made from Paulownia. I have previously built a single and a double from WRC.
I will definitely use Paulownia for most of the next kayak with a bit of WRC for effect.
The samples I weighed suggest that my kayak would have been about 1kg heavier if I had used all WRC. The kayak without seats and footrest weighs 10kg.
I found it about the same as WRC to bend and probably a bit better to plane and sand. It is softer than WRC, but it didn't create any problems in the construction and with the same 125 gsm glass it seems to cope as well as WRC with normal use.
The colour reminds me a lot of Tasmanian Oak when the glass and resin are applied.
I posted a picture a month or so ago on this bulletin board.