Right. You get into the realm of experimentation when you start adding spacers and adapters. Plus it's possible that in so doing you introduce the dreaded "hysteresis" because the caliper's load path to the frame or fork now has some sloppiness in it.

Before going to a larger rotor I'd definitely exhaust the possibilities of pads and possibly bleeding the brakes (I'm assuming hydraulic)

but what do I know ?

/markp