I know this (upper vs. lower speaker terminals) is an old topic (May of this year), but it's been eating at me, so here goes:
SIGNAL PROPAGATION DELAY: At the speed of light, the signal will travel at about 0.98 feet per NANOSECOND, so if the jumper is a bit under 6 inches, the propogation delay is about half a nanosecond. Double this for the comparison between connecting the signal to mid/tweeter via the upper terminals directly, vs. through the jumper from the lower terminals. This means a total distance between the two connection choices is about a nanosecond.
JUMPER RESISTANCE: [Using a Hewlett-Packard current source at 1.00 amp, through a 0.5 ohm swamping resistor in series with the jumper, measuring the voltage drop across the jumper with a Fluke 8060A in high-impedance mode], I see a resistance across the jumper of about 130 micro-ohms. This may not be precisely accurate, but should be well within an order of magnitude.
The other three properties that could come into play here are inductance, capacitance, and imagination. I vote for imagination, and would love to see a qualified person conduct a double-blind test! Maybe there's a big hole in my logic here?