Just because somebody does not have a dyno sheet does not mean that there car has/is not making any more power than it was before.
If somebody pays $xx and feels an increase in power, that may be all that matters to them. They may not be concerned about getting an extra 20hp, all they want is an increase - and they have it. I've made modifications to cars and bikes before, and not had a dyno - but I can still feel an increase in either hp and/or torque. I may have made dyno runs later, and all that has done has confirmed the increase in power/torque - the only difference now is that I have a value of x hp or x lbs/ft tq. Oh yeah, and I've spent $xx for a piece of paper putting a value on my improvements. The dyno is not the be all and end all. I'll agree it is a very worthwhile instrument.