SG is listed as between .80 and .96 at 12%moisture. As this is Welsh timber I would err towards a lower density
Seems to be on par with lower grade osage. Id build a bow to similar dimensions of Mulberry. Its stiffer than osage which may indicate it would be more brittle. Making it wider will help with that, maybe make it a rounded belly. Honestly though, it sounds like an excellent wood to use in a laminated bow, with bamboo as a back. Boxwood has much higher compression resistance than osage. I really think its best as a belly wood, under sinew or bamboo. If you do that, you can make it more narrow than osage even.
What type bow do you think you would rather build?