Skyrocketing home prices across the country have fueled a rise in the origination of new affordable mortgage products with extended or flexible amortization terms. And MBS issuers securitizing those products have had to make some adjustments to assuage rating agency and investor concerns. Take 40-year mortgages, for instance. While the product is being touted as a way for borrowers to reduce their payments and avoid the payment shock associated with interest-only ARMs, issuers are already…