I dunno if it's weaker per gram, but I definitely do know pheno is slower acting compared to pento. With pheno, black out occurs after a couple to a few hours instead of just 1 like pento, and with pheno it takes you up to 3 days of being in a coma before you CTB.
But what the members of Heaven's Gate cult did in addition to that was they put plastic bags on their head with an elastic band around the bag and neck, but they would keep the elastic open with their thumb. Then when they black out, their thumb would slip off from under the elastic, closing the bag around their head so they suffocated from lack of oxygen and died quicker.