I hope some of the photo experts give you some good advice, I am not one however. I am in the same position as you are. I will be following your thread, I'm also looking into a wide angle lens for my SP-350 in an OLY PT-030 housing. Do you think the following WA lense will work?
Fantasea Line | FWAL-01 WA Lens w/ 56mm Thread Includes | 5121