15 is GREAT, not poor. Even a fresh rebuild will have 15-20% on leak down test. And high leak down % doesn't automatically mean motor has to come apart; leak down should only be used to determine where leak is, ie exhaust valve (no big deal), rings ( unless very high no big deal) and intake ( if high enough may warrant a look). Proper use of leak down test or compression test is comparative; so unless you have a baseline from when engine was new and perform test with same gauge same way at same operating temp it's all but impossible to tell just how much wear you have. That's why blanket statements on what readings are ok can be very misleading. Engine temp and even what pressure you perform leak down at can influence the outcome; as will your gauge or buddy's gauge, we would always rock crank back and forth a little while at top dead center with test pressure in cylinder in order to properly seat rings as rings need that pressure to properly seal, these variables will influence out come.
Again though 15% is great for these engines a rebuild is not called for! If test was performed properly your motor is very healthy.
