Feat Question: Storm Bolt

[Faerynn]

Okay, so I have a drow wizard that took the Storm Bolt feat as a way of having a fairly decent attack that never runs out (sort of like the Warlock's eldritch blast) and can be used in a crunch. The Storm Bolt description lists the spells that "make it work" and that the damage done by the Storm Bolt is equal the highest level memorized spell of the listed spells. So, as a wizard who can cast 5th level spells, I had my drow memorize Arc of Lightning (5th level)... so naturally, my Storm Bolt should do 5d6 damage, however, it only does 4d6 damage.

Now, I think I know the reason. When I look at the description of Arc of Lightning, it is Sorcerer/Wizard 5, Innate 4. It seems to me that it is using the lowest level for damage, but I can't be certain. Just wanted to know if a DM or Luna can expound on this issue, if there is even an issue.

Also, is it possible to get an 8th or 9th level spell for Wizards that are "electricity" based to work with this spell? I think a wizard will actually top out at 6d6 for Storm Bolt. I think all the "good" and "high" level spells (ie. Storm of Vengeance) for Storm Bolt are all Cleric type spells.

Thanks.