In a small bowl, soak porcini in the boiling water until soft, about 20 minutes. Working over the bowl, lift out porcini, and squeeze out liquid. Coarsely chop porcini. Pour liquid through a fine sieve into another bowl, and reserve.
Cook bacon in a large skillet over medium heat until crisp, about 5 minutes per side. Drain on paper towels. Pour off rendered bacon fat, reserving 1/4 cup. Wipe skillet clean. Coarsely chop bacon.
Return 3 tablespoons bacon fat to skillet. Add leek and garlic; cook over medium heat, stirring occasionally, until trans-lucent, about 2 minutes. Add fresh mushrooms and porcini, then the salt and pepper. Raise heat to high; cook, stirring, until mushrooms are tender, 5 to 8 minutes. Add thyme; cook 2 minutes more. Transfer to a plate; let cool.
With an electric mixer, whisk cream cheese in a medium bowl until smooth. Gradually add sour cream; whisk until smooth. By hand, stir in mushrooms, three-quarters of the bacon, scallions, and 2 tablespoons porcini liquid. If necessary, add more porcini liquid to reach desired consistency.
Garnish dip with scallions and remaining bacon. If desired, cook sliced mushrooms in remaining tablespoon bacon fat in a medium skillet over medium heat, stirring occasionally, until golden brown, about 2 minutes. Scatter over dip. Serve with waffle chips if desired.