Firstly, credits to cpa4989 for bringing this situation up. I tested this situation with the Essene Poyo with Mindbuster (non-crits only). Tested this with 4 situations (wearing same gears throughout, except when mentioned otherwise). Blue Spritz, Dragon, and Priest Pearl all offer +15 INT (and I chose Def Spritz and MP Dragon to avoid Magic % being involved in testing).
|#||No Mount||Blue Spritz (Def)||Blue Spritz (Def) + Priest Pearl||Dragon (MP)|
Even with the small sample size of 10 non-crit Mindbusters, the differences are obvious above. So as clearly seen, the +15 INT from the Blue Spritz isn't truly applied (same damage as if you had no mount).