Using a spacer is no different than using a wheel with a more aggressive offset.
I noticed no difference whatsoever in steering feel or responsiveness with 12.5 mm F/R spacers on my ZCP M3.
I did notice a very slight decrease in the tendency of the back end to come out with DSC off.
For track purposes, if you increase the front track relative to the rear you will decrease understeer.
From a purely scientific standpoint, widening the track will increase the maximum lateral acceleration your car will be capable of before breaking traction.
Basically, I have found no downside whatsoever to spacers, and they do make the car look better. Just get a good set and make sure they are properly installed.
2014 Audi S6 * 2015 Audi S3 * 2015 Porsche Cayman GTS
Former 2011 BMW M3 owner