There was a 4x fatal PA28 crash coming out of Sandown on the Isle of Wight, a few years ago. The AAIB report reported, from memory, a 25% loss of valve lift and a 10% loss of power.
The average renter would probably not notice a 10% loss of power especially as it shows up as only a ~3% loss of RPM on the runup, which could easily be masked by wind passing through the prop.
It should be apparent to a regular flier who flies by the numbers.