If botnets are predominantly 32 bit (I don't know, but this claim is often made) then they are primarily older computers as well, with smaller caches, often no AES-NI, and (on cheaper models at least) fewer cores. That is much more than 2x. Together it is closer to 10x.

The numbers I've seen are that roughly 50% or perhaps slightly more of Windows 7 installs are 64 bit and Window 8 installs predominantly are 64 bit. 32 bit computers are going to have a high concentration of Windows XP (i.e. old, mostly corporate) or Vista (still hard for me to believe anyone ever used that, but there is a percentage out there).

I don't doubt there are some botnets that target higher end gaming computers, but we don't have numbers. The article cited prices "per 1000" but we don't know how much that can scale. 10K computers for example, would only serve to further secure the network, not attack it.

Without further data I remain unconvinced that botnets are frequently high end systems with good GPUs. And of course if they are then the whole argument of GPU mining being GPU-resistant is completely wrong. Even then the size distribution matters a lot.

EDIT: There is another factor I forgot. Since the botnet has to evade detection it will run at a lower duty cycle, only mine when the computer is even turned on, and suspend or slow down mining while the computer is in use. This likely reduces efficiency over an intentional miner by at least a factor of 2, but perhaps 5 or more (if the computer is powered down a lot).