According to a report from XDA Developers forum, some Galaxy S8 and Galaxy S8+ models are using different storage chips, UFS 2.0 and UFS 2.1, to be precise. To make things worse, Samsung has quietly removed the UFS 2.1 from the Galaxy S8 specification list.
This is not a major issue as we are not looking at a much slower eMMC 5.1 storage chips but some models will have faster storage. According to the same report, Galaxy S8 units with Qualcomm's Snapdragon 835 SoC use slower Toshiba UFS 2.0 chips while the Galaxy S8+ with Snapdragon 835 use Toshiba's UFS 2.1 chips. Most, if not all Galaxy S8+ and Galaxy S8 with Exynos 8895 SoC use Samsung's UFS 2.1 chips.
The Galaxy S8 that was torn down by iFixit.com also had Toshiba's THGBF7G9L4LBATR 64 GB UFS 2.0 chips.
This also suggests that Samsung intentionally wants its own SoC to come on top in some benchmarks. This could also backfire as some upcoming smartphones that will be also based on Qualcomm's Snapdragon 835 SoC could also end up to be faster than Samsung's Galaxy S8/S8+.