18 GPH is maximum allowable fuel flow for an O-360 at 2700 RPM at sea
level and standard conditions (higher fuel flows work, but cause some
loss of power).
16 GPH is maximum allowable for an O-320 under the same circumstances.

These are maximum values; minimum values are as much as 13% lower. 16
GPH would be within tolerances. 15.5 would be a bit on the low side,
but only VERY slightly, as in 1% low. It's not rare for a JPI to be
1%-2% off on fuel flow.

Some other things you should consider:

Are you at sea level? Fuel flow should decrease as air density
decreases.

Are you really making 2700 RPM? Unless you've checked with an
electronic tach, you don't know. Mechanical tachs are notoriously
inaccurate.

Michael