salomon bindings / burton boards

I need some help. i'm running salomon medium spx5 bindings on a burton
custom. i wear size 9 boots. i have noticed that to get any response
with heelside turns i have to crank my forward lean all the way down
on the rear binding. my toeside turns are quick and snappy but the
transition to heelside is a little slow and awkward feeling. i feel
like my boots are centered too far towards the toeside edge of the
board which makes toeside turns instantaneous but heelside alot more
effort, in other words just the slightest toeside pressure gives me a
quick response (what i like) but heelside requires the movement of my
whole leg (and body) just to get the same kind of resonse. the disks
that came with the bindings won't allow me to shift the bindings
towards either edge only front to rear. my sons burton mission
bingings allow side to side adjustment. does anyone know if i can get
some replacement discs for my spx bindings that will allow for side to
side adjustment on the burton 3 hole pattern? i have salomon boots and
i love these bindings and want to keep them.
joe