RB you have already seen the effect of longer exposure at low ISO here
http://members.optusnet.com.au/~cheekyfish/NGC1763.jpg
That is a much dimmer nebula than the Lagoon. Notice no discernible noise in the background and very dim objects are clearer. There are others. One here
http://users.bigpond.net.au/avandonk/LT_s.jpg
Bert