I don't get this uneven development argument at all n i have been developing film for 50 years with perfect results.
I can understand if your development time is very short, less than 3 min, then 15 seconds may impact eveness. Hey, come on here, it taskes the gelatin about a minute to absorb the chemicals n get to work.... It's not an imediate on contact reaction.
But if you are developing for longer times, 15 seconds is nothing to fret about and won't make any difference in the final result.
Uneven development?... Dilute your developer a bit.