As frost I consistently reach the exact same number as my simulations (5% deviation at most). Is it good or am I supposed to be higher than a simul ?
Second question, is there a simple way to check my simul DPS with another spec and optimization ? I'd like to know how high I would simul in fire but really I don't want to reforge regem everything just to perform a simulation.
Question is: What kind of simulation? If you use simcraft, what settings do you use?
Generally, if you simulate single-target DPS with optimal play, your simulation should represent pretty much the best-case-DPS in that scenario. You can only really go higher than that if you have multiple targets, or encounter-mechanics that boost damage in some way (or really good RNG). If you reach your "simulated" DPS you should be doing quite fine.
To find out how well you could do in another specc: We really need to know what tool you are using It is usually easily possible to change gems/reforges in a simulation without actually doing that ingame.