Enhance Potions (EP) mod will only boost the effects of the potion you drink. It must be worn by the drinker at time of drinking. Example, if you have on an Ecru citrine ring with the 50% Enhance Potions mod (which is the cap, used to be 75% untill it was reduced), then you drink a greater strength potion, you get +30 strength stat bonus instead of the regular 20..... (20 x 50% = 30). Or you can wear a ring with 25 EP & bracelet w/25 EP. So at 100 strength you can end up with 130 strength for a little while! Same with Dexterity. Healing is also boosted. I've noticed no better Night sight or no better Invis potion effects with EP. (wish it made them both last longer).
It does not have anything to do with the crafting of the potions. You need to use an Alchemy bonus talisman for better chance at crafting. That link that the previous poster gave is helpful if you plug in your Alchemy skill + any alchy talisman it shows you your chance of success.
Hope that helps.