#1: No, increasing the latency of the time it takes to post spam has nothing to do with the throughput.
#2: Probably not a good idea. It's very hard to get lots of karma from comments alone, and you start to cut in on actually useful posts, like a new founder posting a link to his/her company at launch.
Re #2, it'd only take even needing 4 or 5 karma to dissuade the majority of spammers. And hard? The majority of my > 10k karma comes from comments - possibly even over 90%.
It's not far fetched to expect people to have posted one or two comments a couple of people vote up before being able to submit and it'd decimate this problem.
#2: Probably not a good idea. It's very hard to get lots of karma from comments alone, and you start to cut in on actually useful posts, like a new founder posting a link to his/her company at launch.