Well there is only one (normal <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />) way to enter the dungeon under stormfist castle. I�m not sure if you get extra XP when asking in the thieves guild, but otherwise there is no bonus. You can�t enter stormist castle through the main entrance any more, after thrown out. <img src="/ubbthreads/images/graemlins/badsmile2.gif" alt="" />
So if you play in the usual manner you won�t enter stormfist castle any more, but you will enter the dungeon in the castle�s underground.