I'm interested in relocating a toilet to the other side of a second floor bathroom. This would entail a 3" drain with a developed length of about 8 feet back to the existing soil stack/vent. There would be no easy option to install an additional vent in the toilet drain anywhere near the closet bend. A new shower drain (with its own separate vent) would connect into the toilet drain several feet downstream from the toilet.
Local code incorporates both the IRC (2006) and the Illinois Plumbing Code.
The IRC seem to allow any length for an "unvented" toilet drain, if at the stack there is a sufficient vent.
The Illinois Code (sections 890.1470 and .1490) seem to require a vent within 6 feet of the toilet.
Do I need to figure out a way to add a vent to the toilet drain line in order to comply with Code? If not, would adding such a vent improve (or possibly degrade) the performance of the toilet in terms of the strength of the flush or the risk of blockage building up over time?
You must follow the Illinois code as it is the minimum standard set by the State. You can always be more strict, or go above and beyond the code but nothing less.