Yea I just checked and I have around +160 spell damage (mixture of + frost and + magical damage and healing). I guess this seems to add up for Frostbolt but not nearly as much when we look at Cone of Cold. I do faintly remember spell power benefitting cast time spells better but it's been so long since I played original vanilla I'm not sure.
p.s. I just did a quick test - I tried a few fireblasts with -55 spell power averaging out at about 280 damage. Slapped the spell power back on and they went up to around 300. I tried the same thing with Frostbolt with the first casts averaging out at around 330 and after slapping the spell power back on they went up to around 400. So I'm not sure if the spells Cone of Cold and Fireblast are not as strong because they are instants or if there is another reason behind it.