OK, have been playing through Bgtutu with SCS+Grey Clan Ep 1 installed, solo & on insane, and am a shade below the BG1 level cap with my Geo Sorc.
So far so good. Sleep+wave spray was a useful combo for mobs during the first couple of levels. As with a normal sorc, the breakthrough comes with getting fireball, and being able to take out the groups much more easily. With resist fire I can also cast fireball on myself when surrounded by enemies [incidentally, I thought fireball was a "magic fire", but prot from fire seems to protect against it completely..even though it says it only gives 80% prot against magic fire. Odd.]
Balance - it's a bit overpowered overall, but this is more due to the "sorcerer" elements - eg. being able to acquire stoneskin in BG1 (yeah, I couldn't resist picking it!) - rather than any changes the Geo Sorc kit makes. In terms of the kit's disadvantages, the 15% XP penalty is pretty meaningless in a solo game, as you might expect. The stat penalties are a minor pain for a spellcaster. The biggest problem is the increase in the spellcasting duration, which can make the difference between getting a spell fired out at your enemies and being cut to bits. Certainly it has made me a bit more circumspect in casting summon insects, which is a great spell but carries a long casting time.
Onward to battle and victory etc. Will post another report once I am done with Tutu.
coaster