The Dark One wrote:
My question is this, what sort of performance hit does my forum registration suffer as these lists get bigger. Are legitimate people trying to register having a noticeable wait while the e-mail domain is checked against the list?
Since you asked about what the performance hit is....
Those lists are only checked once per registration and once per session start (that is, after a login), so the performance hit is in ms, can't be detected by a human. Overall, computers can scan lists pretty fast.