Tome of fire boost the original amount of damage by 50%. The maximum amount of damage a mid-level account can do with chaos gauntlets and Tome of fire is 22. Adding in Occult, boosts it to 24, Staff of the dead boosts it to 25. Using both Staff of the dead and Occult makes it a 27. If you add in the Ancestral robes set bonus you can push Fire Bolt to 28. All of that on top of the Slayer helm (i) would make your max hit a 31 on slayer targets.
The Magicexperience gained depends on the damage dealt. The amount of experience increases by 2 for each additional damage dealt as shown on the table below.