First off....there's no need to get snippy here folks. This is just a debate.
trinity_gt, I believe your "sound engineering philosophy" may be flawed. If Modular engines are so sensitive to changes in crankshaft harmonics, why is it that all factory Mod Motor crank pulleys...whether they be of the 6 or 8 rib variety, fitted with cast, or forged crankshafts are all, and have all been interchangeable between several varieties of the 2V, 3V and 4V 4.6L, 5.4L and 6.8L engines?
Contrary to what you might believe, the stock modular damper/pulley isn't "precision built" with quality or even great accuracy in mind. I've seen much better craftsmanship and deeper thought put into the design of most aftermarket pulley set ups than the factory damper/pulley combination. The very fact that the stock set up is cast and pressed fit together into a rubber ring, with "best guess" timing accuracy in comparison to the machined billet/steel and balanced precision of the aftermarket versions is more than enough proof of this.
I’ve been running a stock 8-rib 5.4L truck lower damper/pulley on my forged 4.6L 2V for going on 5-years now without a single issue. If the factory pulley was so balanced and precise, I should be experiencing all sorts of issues with vibration, bearing wear and even oil pump failure? I can also tell you with great certainty that the precision machined and balanced two piece damper/pulley design and construction of the Summit brand pulley on my current 5.4L 3V is light years ahead of the stock cast, press fit factory pulley that it replaced.
Also, “piggyback” style pulleys should be avoided. These are what are going to cause problems IMO. Not so much because of their craftsmanship or the balance of the new underdrive portion of the pulleys themselves, but because they’re dependant on the accuracy and condition of the factory crank damper/pulley they’re now combined with. Any imperfections or wear (i.e. rotted rubber ring/separation or slip the stock pulley may have experience over the years) that might be present in the factory damper/pulley are going to be magnified when you add the weight of a new underdrive pulley and move said weight outward from the centerline of the crankshaft.
I'd prolly add that money to like a Mazier (sp?) electric water pump or something. Not only will it likely reduce parasitic drag more than a 20% underdrive (just a guess) but I've always liked the idea of circulating coolant through the motor for 15 - 30 seconds after engine shut-down.
Why not add both pulleys and the pump? I did.....and it made a noticeable difference in pep and acceleration (as well as fuel economy) in my 6,000 lb F150 FX4 Supercrew, so I can just imagine the difference it would make in a Mustang weighing nearly half that?