Would think that Faint Attack would be a better choice for a second attack than Shadow Ball. Unless the pokémon in question is Adamant or something. STAB gives it effective power of 90, vs 80 for Shadow Ball, but you get more value out of ignoring Attack and training up special attack for both Faint Attack and Solarbeam.

Unfortunately if you've evolved it already, you can't relearn Faint Attack.

The Sunnybeamer moveset I'd suggest, if you're working with a new Seedot, is:

Sunny Day
Solarbeam
Synthesis
Faint Attack