[Fat Charlie]

I can't think of any real safety problems with dropping the front and not the rear, but if you've got ABS or any sort of traction control then those systems might not be happy with running multiple tire sizes.

Aero-wise, the only way to find out if it helps is to thoroughly examine what you've got now and what you end up with. Dropping the front but not the rear might only increase your frontal area without giving any corresponding benefit. "Downforce" is just drag and is only useful if you corner at such high speeds that your car would fly off the road if your bodywork weren't sucking it to the road. Since you generally can't turn downforce off, that drag is working against you every moment that it isn't required to keep you from flying off the road. You may also need a belly pan and/or side skirts to actually get the effect you want.