You end up losing all around because of the velocity drop, a four hole spacer that extends to the manifold carb flange is the best option. You might think that you could have the best of both worlds with this setup but because of the restriction caused by the dual plane design you never get the gains of an open plane design and end up losing bottom end because of the velocity drop in the spacer...it actually will cause fuel drop out on some installations because of this. One thing I have learned over the years is to avoid changes in velocity in the intake system because when fuel is mixed at the carb any change in velocity (slower) can cause fuel drop out. the exception is when there is a temp change, hotter air/fuel can handle a velocity change better than a cool charge can. Sometimes a HP increase (mild) can be seen on a dyno but it almost always is accompanied with a loss in torque at lower speeds. The end result is less area under the curve which means less total power and a weaker engine overall. Abrupt changes in velocity must be kept to a minimum no matter what in the intake system particularily with carburators. Some times a poorly designed manifold with bad distribution can be helped by this type of setup but these days the manifolds are so good it does not apply anymore.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo