Also its been years since I last had SoA installed on its own but I think you still gain exp after hitting the cap so you can level up straight away after installing ToB. Also just for reference once you have ToB installed the SoA exp cap does not apply to SoA any longer and you can reach 8m at any point.
I think if you have ToB installed, it raises the level cap, even if you are in SoA. If you just import an SoA character after installing ToB, your exp will be bumped to 3 million, hence the probably immediate level ups you're getting.
The pain will only be passing. You should survive the process" A really bad pickup line in Baldurs Gate!
The first time I played BG2, I didn't even know there was a level cap. I was totally bummed after getting all excited waiting to cast level 9 spells only to discover when I hit the cap that you can't get a mage that high in SoA (I was playing with four characters). However, with ToB installed, the level cap for SoA is effectively gone and you can memorize and cast level 9 spells in SoA. It certainly makes the end game a bit easier.