RMAN Overhead on the database
That depends on your disk subsystem.

A full backup is going to involve a metric crud-load of I/O (that is a technical term).  That is going to put a huge load on your I/O subsystem.  Depending on how I/O bound your system is, how much spare bandwidth your I/O subsystem has, how fast your I/O subsystem is, etc. the impact will range from "yeah, maybe it's a bit slower" to "everything times out, the system is unusable, users will be visiting the DBA team shortly with flaming pitchforks to express their displeasure."

I am hard-pressed to imagine a whole lot of situations where I'd ever want a full backup running on a non-trivial production database during anything close to business hours.  If you have a real need to do this, I'd tend to be investigating things like DataGuard that would potentially allow you to offload the load of doing the backup to the standby database.