I have been grafting asynchronous support into my codebase by a) writing new code with it and b) using AsyncContext [1] to run asynchronous stuff synchronously.

The performance:lines of code ratio you can get with async is quite high. Using ADO.NET you can run mutually exclusive queries in parallel for up-to a 2x speedup (depending on what the queries need to do):

    using (var connection1 = ...)
    using (var connection2 = ...)
    {
        var getSomething1Task = GetSomething1Async(connection1, ..., cancellationToken);
        var getSomething2Task = GetSomething2Async(connection2, ..., cancellationToken);

        await Task.WhenAll(getSomething1Task, getSomething2Task);

        // now aggregate results from 1 and 2
    }

And it's more or less the same lines of code you'd need for the synchronous version.

[1] http://stackoverflow.com/a/9343733/242520