I've debugged a further issue today with the latest version of Pro Sites and using PayPal. It seems there is a race condition between paypal ipn and the form loading again after signing up to a subscription. Success messages are only outputted if there is a gateway in the pro_sites table.
This can be seen in lib/ProSites/View/Front/Gateway.php:L239 the problem is IPN doesn't seem to be instant and therefore there is never a record in here for when a new subscription is created and the form is reloaded. Therefore ::get_existing_user_information is never called which means the success messages are never shown.
Could this be resolved quickly as currently its possible for users to double(triple etc) signup because the page basically remains the same after submission. Note we're using the cc_paypal_checkout