IPN is generally pretty much real-time. There are times when it gets a little lagged and can be kind of slow, but it doesn't seem to happen all that much.
That said, the API calls along within your checkout will indeed return lots of good info in the actual response. Your URL only has the parameters you mentioned (token and Payer ID), however, the GetExpressCheckoutDetails response would have all the buyer info you need, and DoExpressCheckoutPayment will return the transaction ID, payment status, payer status, etc. So the DECP probably has the majority of what you're after.
You can use session variables to save GECD response data and DECP response data accordingly and then update your database, send out email notifications, etc. after calling DECP.
The thing is, the payment could end up being "pending" for various reasons. As such, IPN would still be the best way to handle this sort of thing so that you can update your DB and send out one notification for the pending payment being received, and then another update/email once that payment actually clears.