I find they stretch a bit, maybe a bit more than the Magos (if that comparison is any use to you).
I wear a UK 9 / EUR 43 in an Anasazi velcro and a UK 9.5 / EUR 44 in an Anasazi white. The Instincts would be EUR 43. When I compare with either of the Anasazis they are significantly shorter, approx 1.5CM in the case of the whites which have been perfect fit-wise. No wonder I can't keep them on for long!
I'm pretty sure I tried a pair of broken-in Magos earlier this year and managed to get a 41 on which was tight but acceptable. I then tried to buy the Magos but the 42.5 and even 43 killed me / just felt wrong. I have a feeling that both the Booster and Mago will allow you to get on way smaller than normal boots and still wear them but all you're doing is stretching the shoe out of shape i.e. the diagonal band of rubber running from top back of the heel to mid-sole gives too much and the shoe just ends up really splayed if that makes sense.
If I was going to fit in any Mago it'd probably be the 43.5 in which case a 43 in the Instinct, based on your experience, makes sense.
Cheers for reply.