Seems the default is 20, currently I have mine at 40 because new discussions just fall off the page in a few minutes. Can I increase this more? Are there any serious down sides to having 60 or more discussions per page?
Other than hitting the DB with a higher load for each page load? Not much probably. If you got the resources to throw at it, or the lower traffic to support it, you probably won't have any major issues... but personally.. I'd probably not do over 30 (think mine is at 25, but it may still be default).
Otherwise, look to the "never-endng story" possibly of non-stop scrolling. I'm not sure, but I think at one time there was an add-on that allowed that.