you should control your estrogen, not kill it during cycle.
this attitude that you need to run letro, nolva, clomid, aromasin, adex and every AI under the sun during cycle is nonsense and YES it will hurt your gains. you need to experiment and find the right dosage for YOU.
i have found forma to be the best AI out there because you can adjust your squirts as you see fit. as you gain experience and learn to understand your body you will find the right dose.. newbs seem to struggle with it because of lack of experience or some guys are just clueless and run roids for 20 years and still dont' have a clue about their own bodies cause they are stubborn and don't want to learn from each cycle. no different than someone who goes fishing everyday and doesn't catch anything but insists on using cheese as bait and using a shark hook to catch 10" fish. learn from your mistakes and find the best way