Anyway, I'm replaying through Book I with my second melee-fighting Healer - this time focusing on spell 'buffing'.
So I decided to see if potions like Nimbleness and Ogre Strength "stack" with their spells.
I had earlier cast a Level 6 Nimbleness spell, which gave me DEX=40 (from original DEX=10 + 30 (from the Level 6 Nimbleness spell)).
Just to see what would happen, I then drank a Nimbleness potion (which I had mixed myself!

And, lo and behold, instead of getting boosted up to DEX=65 (from original DEX=10 + 30 (from the spell) + 15 (from the potion)), instead my DEX was cut down to DEX=25 (from original DEX=10 + 15 (from the potion) + no spell bonus!/spell bonus "overridden"!!).
I haven't gotten a chance to see if the same thing happens with Ogre Strength, because I haven't had a chance to buy the spell yet. When I do get the spell, I'll test the 'stackability" of that potion & spell as well.
Anyway, I just thought I'd report this.
(I have a dim memory of this being discussed around here before, but I don't recall whether BW said this 'lack of stacking' result was intentional or not....)