An area I can see slip-ons giving a squeak more power is the "scavenging" ability of getting spent exhaust gas out of the pipe.
As we (mostly) know, the stock muffler is a series of switchbacks to dampen the noise of the engine, in turn impeding the flow rate out of the exhaust. This would potentially result in not all spent exhaust gas leaving the pipe and in fact a small amount would be sucked back up into the engine on the return stroke in that moment before the exhaust valves close*.
Slip-ons are typically straight through mufflers surrounded by baffling material, so I would theorise the exit velocity of the exhaust gas would not be as impeded by a slip-on compared to the stock muffler, in turn resulting in slightly less spent exhaust gas being recycled back into the engine, in turn giving a little more power.
This is assuming the diameter of the muffler is not so large that it has a detrimental effect on the velocity flow of the exhaust gas (imagine a water hose that is too big or small and what happens to water that either squirts or dribbles out the end).
So on that basis I personally believe claims of small HP increases with slip-ons alone - however - it is difficult to quantify at the wrist due to 5hp /155hp = 3.2% gain in power. So it could be hard to detect (I believe) whether you have more power, or are just revelling in the joy of snapping open the throttle to hear the big girl roar.
I'm far from an exhaust expert, but used to sell cruiser exhausts and got a base level understanding of scavenging, reversion, velocity, sound/pressure waves, etc.
* note: as far as I am aware no consumer grade engine is capable of completely eliminating all spent exhaust gas before sucking in fresh gas for the compression stroke, so it is a matter of how much extra spent exhaust gas is scavenged over stock.
Happy to be contradicted on any of the above, more my musings on the topic really.
