I'd say that the majority of proactive Tunnel strategies are rather slow. If you have played enough Hoard games you realize, though it is a strong card and often useful, that Hoard can actually slow down VP acquisition by the tune of -$1, at least initially. The Gold you gain goes into the discard, which is about the worst place it can be. Tunnel is a dead card in hand, even with a way to discard it; is not worth even the $2 that Hoard provides.
So, it requires more than one activation of each Tunnel to gain a measurable improvement of deck economy. The first Gold gained average out to $1.5 value added to the deck, less than a Silver. The second Gold improves the average to $2: two Golds + Tunnel = $6/3, a Silver. You can also add the 2VP as a measurement to the value, for what it is worth, and that 2 Gold + Tunnel may be better than 3 Silver at getting to $8 hands.
As far as specific proactive strategies, my opinion is that Embassy is about the best one. It cycles like mad and Tunnel is a perfect discard. Virtually every activation of Embassy improves the buying power of the deck and the buying power of the hand that it is played in.