used for a guitar only (and not for vocals), there wouldn't be critical moisture around anyway, so it would be safer to leave the foam in place, instead of constantly opening up the mic, with the danger of damaging the diaphragm.
Still, at 344m/s speed of sound, and a distance of, say, 3.4cm between capsule center and circuit, we'd be talking about reflections "delayed" by ~0.1ms; further reduced by oblique sound incidence with typical mic positioning. Did you try placing the microphone at various angles (and distances) to the guitar?
Best regards, Martin Schneider / Neumann Mic. Development