Check out Mitch Steele's book on IPA regarding this, if you haven't already. According to the book, hops definitely do have a preservative effect, but that's not why they were added to beer going to India. They were also added in the same amounts for domestic beer. Apparently the term IPA was invented for marketing long after the style was being shipped to India, and the style was not unique to beers being shipped to India. There was a difference in that the beers being shipped weren't aged as long before shipment, compared to the equivalent beers being sold domestically, and some thought or evidence that the constant rocking of the casks during shipment hastened the aging process. So they could have and probably did taste different when they were leaving England on the way to India, but tasted the same as English equivalents once they were in India.