ΔE 25.4, loose: different enough that you should pick on purpose. Imperium Blue is noticeably darker, more saturated, warmer than Sapphire Gem.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Imperium Blue is noticeably darker, more saturated, warmer than Sapphire Gem.
The same difference applies the other way round: Sapphire Gem is noticeably lighter, more muted, cooler.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Sapphire Gem on the colour wheel See Imperium Blue on the colour wheel
All matches: Sapphire Gem equivalents · Imperium Blue equivalents