Two final things. One yes spell power can be extremely...well..powerful. especialy if you spend all your high arcana on it(something I cant even imagine doing but I know many do). However I really feel that once you get into those higher levels(14, 15 16 etc) theres lots of baddies with very high saves and/or SR...so it sort of makes up for it..I personaly would think.
The Red Wizard. lets talk about the Red Wizard. Firstly, you have to be a human, from Thay. You cant be good. those two things already make it seem not very PC classish, to me. Nextly..if my realmsknowledge is right, as a RW your very very tied up with the politics and running of Thay...and your subject to the Zulkirs. it also seems like it would be a massive stigma considering large number of people in Faerun at least distrust the RWs and many, many consider them sworn enemies.
Put this all together and personaly if I was DMing...well a player would have to have a pretty darn good RP reason for taking that class, and there would be some pretty heavy RP/storyline drawbacks.
I just cant imagine wanting to be a red wizard myself..I mean there the bad guys for bleeps sake(I know there not all "technicaly" evil but still). and..the Simbul!!! Theres more than enough reason right there not to become a Red Wizard sheesh...
back to spell power briefly...rule that it doesnt stack. or say, you cant take this ability more than twice, or something simliar.
I think that covers it for now...
