Would refilling an AC with refrigerant improve its performance after 8 years?
how likely is it that some of the refrigerant has leaked over all
  those years?


Not very likely.

It's a pressurized system, so if you had a leak I would expect an 8 year period to be more than long enough for ALL the refrigerant to have escaped. There is a slight possibility of a seal partially failing and only letting refrigerant out when the unit is operating (pressure is greatly increased), but you would know pretty quickly - the cooling capacity would continue to decline as you use it, until it has little to no effect on the ambient air.


  Would bringing it in for a refill likely make a difference?


That depends on the current charge level. An under/over charged unit will not perform to the manufacturer's specifications. Not having any leaks doesn't necessarily mean the system has the correct charge in it. But it sounds like you're talking about a portable unit that has probably never had any maintenance performed on it (so it should still have the factory charge, which should be correct).