If you have to site every single blow of the chisel, regardless of how long you've been at it, you might as well get a mortising machine. That, or be cross-eyed by the end of the day. My head starts pounding just thinking about it. My favourite mortise chisel is a Marples 1/4" that's probably about half its original length, maybe less. 5/16" isn't far behind. Easier to hone at that length, too.
I'd love to have all the wood that was likely thrown in the stove by people who didn't know how to quickly and effectively fix a mortise that was chopped slightly off plumb. I'd be rich. You know who you are. Just buy more wood.