"When you cast a spell you can decide to increase your caster level by 1, 2, or 3 but you are stunned for an equal number of rounds immediately after doing so. Your increased caster level affects all level based variables of the spell, including range, area of effect, spell penetration, and the difficulty of dispelling the spell. You can exceed the normal level-fixed limits with this feat so a 9th level wizard could use reserves of strength to cast a fireball as a 12th level wizard and deal 12d6 fire damage. If you are not subject to stunning effects, you instead suffer 1d6, 3d6, or 5d6 points of damage when you call upon you Reserves of Strength feat."
Normally, it would seem that people would just seek become immune to stunning. However, my question is can you break caster caps with this feat even if you don't increase your caster level? For example, if a 20th lvl caster uses fireball but doesn't use the feat to increase his caster level above 20th, does his fireball still do 20d6 with no kind of backlash?