I currently use Salomon Synapse boots, and while they perform really
well, they're a little short front to back, such that my toes are
hitting the end of the boot unless my knees are bent and my heels back
in the boot. My current boots are a 27.5 and I've had my foot measured
and I fall right between 27.5 and 28. I'm planning on buying Salomon
Malamutes here shortly. So do I go for it in a 27.5 or a 28?

I would go for the 27.5 and mold the liners with a toe cap on. My feet are
26.7-ish and I ride the size 26 Malamutes from 01/02, and have ample toe
room after molding the liner w/ a toecap. I also use a custom footbed
which improves things hugely, keeps my foot from moving around in the boot
without having to tighten it too much.