If you are in doubt, just approach the mechanic they used to fix it. Don't see why you can't or shouldn't do that........although the mechanic might be connected to the dealer somehow and might give you dodgy answers (if and only if they agreed to fix up your car dodgily)
As for feeling slower than before, have you check your engine oil dipstick? If they have change the oil and overfill it, it will cause the same effect. Most likely not the case, but hey it cost nothing to check that yourself 
Last option, is as what o0 Matt 0o said, get your trusted mechanic to go over the car (although this does not guarantee they will find problems that's there)
When in doubt, power out....of control.