Obviously, the higher the MP, the more noise you are going to get at high ISO, or at least the more the noise will be magnified.
Your statement make no sense!
The number of MP itself will not affect the amout of noise but the higher MP sensor will give a higher image quality (less noise vs resolution) than a lower MP sensor at high ISO because of the efficiency of nose reduction algorithms.
This is often repeated but to me the comparisons between the new Sony 24 MP crop sensor and the old 16 MP one seems to show that it wasnt the case. When the former was down rezed to the latters size it was clearly still the noiser of the two.
Personally I'd be looking to buy a high MP body if I did upgrade to FF but I do question how much of a market there is for that. The really high end studio/landscape users will go MF, the wedding photographers will I'd guess want more ISO first and will the typical amature user print 30X20 to make use of such resolution? will he be willing to deal with diffraction becoming a big issue?
To me if the the 5D mk3 stats we've seen are correct then it does seem to hint that the D800 has caught Canon a bit on the hop(not now but whenever they discovered its stats) and has convinced them to shift across more of the 1DX features than they were planning to. Ultimately I'd guess losing a few 1DX sales is less important to them than potentially having users switch brands or indeed tempt Nikon users who arent satisifed with a high MP body to do so.