I've been using your firmware and found personally this isn't true at all.
Any details? How many chips do you have on your board? Have you recompiled the firmware with the proper number of chips? What speed are you setting in bfgminer? Have you tried lower speeds so as to exclude the effect of heating? Say, try speed=1000 and tell me if you're getting 1GH/s per chip. Are you getting any error/warning messages from bfgminer? If yes, what messages specifically? That could aid in diagnostics.
Also, have you tried to see per-chip performance? There are some APIs/command line tools that let you see how many nonces did each chip produce. Chances are that you have failed or improperly soldered chips on your board(s).
We now have multiple users using our firmware with proper results, so please be sure to verify that you did everything right.
Can someone confirm that shorter and higher quality USB cables yield higher speeds?
I would consider this highly unlikely. It's like spending a fortune on your HDMI or S/PDIF cables: that won't improve your digital audio quality unless your existing cable setup was completely screwed up.
An easier method might be increasing the resistance at Pin 11 (sense +) through TP1 on the regulator
That also seems to me a bad idea. The sense pins are inputs and they normally draw little to no current, so increasing the resistance will not produce any meaningful voltage drop that is proportional to output current.
Have you had any regulators fail yet?
Not yet.
I favor the method of holding my hand slightly above the heatsinks,
With hand or with a sensor, no matter how good is it, there is a fundamental problem of access to junction temperature. You can't measure temperature directly on the chip die, and that's the only one that matters. Temperature on the heatsink may be substantially lower than that on the junction if thermal coupling is poor between the chip and the heatsink. In this case the heatsink will stay cool but the chip will remain hot. Just imagine a heatsink that is not in physical contact with your chip. The same thing happens when it is in physical contact but for some reason thermal coupling isn't very good. Temperature difference also increases when the amount of dissipated power increases, so your estimation of junction temperature will be increasingly less accurate as the amount of power dissipated by chips gets higher. You could of course do all the math if you have thermal models of your system and you correctly estimate their parameters but well that's a very demanding job to do it right.