No adverse handling effects with a 15/20 combo if you use quality stuff like H&R and install properly.
If all you did was throw 20mm spacers on the rear and left the fronts alone, yes, you'd probably feel more understeer since you're widening the track at only the rear. 15/20 is close to even, so you won't notice the extra 10mm out back in street driving.
Remember those old Pontiac commercials? Wider is better.
