J-bars will most likely solve your problem. Go to a local shop and see the boot-fitter. Let them know you have heel lift and they'll most likely slap some j-bars on the liner for you and you'll be good to go.
That being said... there is probably also a portion of this coming from the fact they are Burton Motos. I'm not trashing them, I rode them for years. But really, they're an incredibly soft and low-end snowboard boot with the most basic of liners.
And THAT being said... even expensive boots sometimes need some tweaking from a boot fitter. Higher price does not always equate to a perfect fit right out of the box.