I wouldnt recommend going higher than 60FPS. Mostly because rarely anyone uses 120HZ monitors atm, so you just waste a bunch of CPU power.
I don't know which resolution you are streaming in, but decoding a 1080p70FPS flashvideo takes approximatly 30% of your CPU.
About the delay: That mostly depends on the twitchserver you are streaming to, but generally you will get 7-8 seconds.