Given that the most read sectors probably contain parts of your OS and pagefile, and considering the size of a modern OS and the size of a modern game, you really expect there its likely with only 8GB that the sectors containing your game will end up on it? Its not impossible, but I know several games that have more than 8GB of content in and of themselves.
But you're missing the point. To maximize the benefit of the cache the stuff that's accessed most should be in the cache, whether it is the pagefile or something else. Otherwise, when your game is running and Windows is hitting the frequently accessed part of the page file, your game will run slow. Likewise, what is the point in putting 8GB of your game into the cache if only 5% of it is accessed frequently? Not only is that poor cache management, it also means that any other processes running that could be making use of cached data aren't because your game data is needlessly filling it up.
As more of these drives come out their cache management algorithm and cache size will be probed in performance reviews. If you want a drive to fully "cache" your game, buy an actual SSD and install it there. Meanwhile, this hybrid approach should provide an excellent cost trade-off for the general consumer, who should see a huge performance boost, despite not having 100% of data residing in the cache - but that's because it's less frequently accessed and thus contributes less to the perception of performance anyway.