I am sorry, but its clear your understanding of 64 bit is rudimentary and incorrect; far and away the greatest benefit of 64 bit code is the way large data arrays are handled, with performance gains in the order of several hundred percent over 32 bit, and large data arrays will be used extensively in a game like Sots 2.
Many 64 bit instructions can also carry out multiple operations per cycle as opposed to the single operation of 32 bit, thereby cutting down the number of cycles per core required to carry out given tasks. The data addressing advantages of 64 bit extend beyond the maximum possible address space, giving access to 64 bit data types that allow the direct manipulation of larger values, reducing overheads that were present in 32 bit calculations. There are no inherent advantages of multi threading in 64 bit over 32 bit, your claim is an over simplification, any gain in performance is a result of gains elsewhere in the code due to 64 bit.
There are more advantages to 64 bit, but in order to explain them, the post starts getting longer and more technical, and the benefits in real terms are smaller; suffice to say, coded correctly ALL programs will benefit from a move to 64 bit, regardless off how much memory they may, or may not access, even where they are not multi threaded.
Almost forgot, the cap on addressing 4GB memory by programs is a "feature" of Microsoft's OS, and not 64 bit architecture, 64 bit processors can directly address a terabyte of memory. The figure is arbitrary, a guess by Microsoft as to how much memory the average program will require, a guess that already hampers developers no end. How much quicker do you think SOTS 2 would be if it had enough memory available to cache all sots 2 resources should that much memory be available on the host system? Please do not confuse this with caching in a ram disk, if sots 2 were allowed direct access to that much memory there are gains and reduced over heads, when compared to speed gains available on a system using a ram disk; using a ram disk is a poor cousin.