If you go with spacers be very wary of all the cheapo ones with a hole through the middle for access to the hex in the pedal axle - leaves silly-thin walls at the undercut at the thread and are a death wish IMO.
This leave solid jobs which should be fine; however note that you'll need pedals with spanner flats on the axles. The best I've found IMO are JTEK jobs which can be had from SJS Cycles (free post if bought on
ebay).
Personally I'd not get too hung up on the magnitude of changes to Q-factor - a few mm here and there is pissing into the wind if you want meaningful change; to give an idea the difference between a road crank with SPDs and an MTB crank with wide flats is about 50mm or 20% on the pedal centres.
Personally I can't ride anything with a narrow Q because it shafts my knees and hips, but everyone's optimum will be different and I appreciate your main motivator isn't comfort in this case..