Will single long exposure achieve the same effect keeping iso constant?
9 exposure with total integration of 2.5secs = smaller aperture and keeping shutter open for 2.5secs?
Unfortunately this will not help.
Basically by doing so you will be just reducing amount of light per time unit to the sensor to keep exposure longer to obtain the same amount of light.
Mutli-Exposure method is different - it allows to capture more light - up 9 times more for 1Dx (and 10 times more for Nikon D800) than in single shot and then averaging that (normalizing) to one single shot image . But random noise is not increased by 9 times but only by Sqrt(9)=3
So after averaging 9 shots in ME sequence (normalizing it to one shot) we will have 10*log(9/sqrt(9))=10*log(3)= 4.7dB image SNR improvements.
On the link below you can find detailed explanations - I was referring this in one of my previous post .
Found it when was searching for something that could answer all the questions.http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm
" In general, magnitude of noise<http://www.cambridgeincolour.com/tutorials/image-noise.htm> fluctuation drops by the square root of the number of images averaged, so you need to average 4 images in order to cut the magnitude in half."