You know what I recently learned? Depending on how you add templates, they can either be an absolute joke for CR, or they can completely crush your party without even giving them a chance for survival.
For example: The Mighty Template from Pathfinder is +5 CR. Gives +5 to AC, initiative, saves, skill and ability checks, +10 to attack, +10 hp per HD (50 min.), DR 1/- per 3 HD, SR = CR+12, +7 to all save DCs, and immune to all mind-affecting, sleep, paralysis, etc. For a CR 1 or 2 creature, that's a heck of a kick start. Next, the Mob template. Turns any creature into a CR 8 or adds +2 CR if it's 7+. Makes it gargantuan, does 5d6 damage, and essentially works like a swarm with 30 HD. If you add the two templates together, you either get a CR 8 creature, or you get a CR 13 creature. Neither of which fits. The CR 13 creature is too easy for a 13th level party, but at the same time it would wipe out an 8th level party.
Which would be more fair? Would you actually get away with sending this against a weaker party? Maybe make it CR 9?