I thought about it, too. It shouldn't be that low for a reasonably sized fan. Still, the problem is that hot air has no way of getting out.
Also, the new fan will be blowing against the original small one, if I'm not mistaken.
But, what the heck I'm gonna try it anyway.
The best solution would probably be replacing original one, but since this is in desktop a1200, that is not an option.