Oh I see. Wireless g will connect at 54Mbps but will max out at speeds around 25Mbps on a speedtest.
So your slowdown from 70 to 50 real world still means the N devices are getting far in excess of a g device BUT there's still a slowdown.
I have a lot of wifi interference so I only get 50Mbps on a speedtest even without a g device. Perhaps that's why I don't see a slow down as you mention with g devices you see 50Mbps. Perhaps I never get fast enough anyway to whitness that slowdown. If that makes sense!!