A well argued  case and after a couple of drinks I might be inclined to go with it, but then again after a couple of drinks I go along with most things    

I've lost count of the number of times someone explained the secret of the universe to me, I believed them all, trouble is it ends with me sleeping on the beach under Brighton pier  while they walk away saying well that was fun   

Trouble is these schemes were always based on some supposition to start with.  Bit like the lost motorist who stops to ask directions - back comes the reply " Well I wouldn't start from here" and the guide then explains the directions from somewhere else to their destination. 

So if I substitute your pixels for bricks what you are telling me is there are more bricks in a ten foot high wall than an eight foot high wall.  Yep,  suppose there is, but the number of bricks per square foot is still the same. Your supposition is that because the picture is bigger from a 400D it will have more noise than the smaller picture from a 350D . But this is not the case, if both were printed at 300 pixels per inch the noise would just be bigger or is that what you mean by more?  

But this is your second supposition because in reality the 400D does not have more noise than the 350D when measured by any scientific means. At all ISO's the 400D outperforms the 350D in noise suppression. 

Ah well it is a good argument - I'll use it to explain why I don't shoot with  Hassy H1D 
 
 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html