I was debating this when i got my slingshot D2 boots. I am a size 10.5 and wasn't sure if id get the 10 or 11.
I finally decided that it'd be easier to make the boots smaller than to make them bigger if i really had to. at the moment of purchase i thought to myself that i could put in some foam of some kind in the toe box if there was too much room.
not long ago i decided i wanted a little less room in the toe box and cut up some old LF Ultra Sucks foot beds and put them inside the boots under-neath the removable liners.
Sooo... maybe you can do something similar if necissary... i don't know. I am probably one of the only guys who have done this?
even better,... I get a li'l more cush!