Yeah, I don't start fretting until until about 24 hours in. Different yeasts behave differently.
By 150% culture I assume you mean it had 1.5 times the cell count of a Wyeast smack pack. Even without knowing the packaging date on the yeast you used, nor the OG of your stout, I'd still bet you underpitched. But all other things being equal I'll still bet it will be OK.