Very specific algorithms can benefit from having wider (and more) CPU registers, but...
1. It's not a very common case that the difference is significant
2. You might have to write the implementation in assembler to have that advantage - when using a higher language (avast! is mostly written in C/C++), it depends on the compiler how it deals with the optimization - and it can easily happen that the 64bit version will actually be slower (some time ago, I did some benchmarks on the decompression algorithms, and for some archives the unpacking was a bit faster, but for some also a bit slower)
3. The executables are a bit bigger, consuming more memory - which can cause slight slowdown, compared to 32bit version
4. The limit for avast! (the weakest part of the chain) will usually be the data source (hard disk, network), not the CPU itself - so faster processing won't really speed up the scanning