If your transceiver has an S-meter, one "S" has 6dB.
Is decibel small? In my sense, yes, dB is quite small: it is difficult to hear a change of 1-5dB in voice quality.
A 5-10dB change is significant, 15dB or 18dB totally changes conditions.
Few devices in the amateur market can measure a 1dB difference.
But precise "bookkeeping" of the antenna system is important: the sum contains many small numbers: +7dB antenna gain, -0.5dB lost in UC-1 connector (other name for PL-259) , 20 meters of feedline with -35dB per 100m gives a 7dB loss, another UC-1 connector: In total, this system has an effective -1dB.
Connector power loss like UC-1 (PL-259) are not measurable at home, so take it from literature (most sources give 0.5dB).
Loss in longer cable can be measured in a good quality amateur testing set, or more simply read it from the manufacturer's data sheet.
In reality small changes can add up: increase antenna gain 1.5dB, install a better feedline for 0.3dB/1m, better connectors etc .... and you've made a 6dB improvement. This is a good result.
FM has a different behavior from SSB: quality is 'good' in a broad central part of scale (hard to distinguish for an untrained ear), a small drop in quality (voice+noise) near the low range, and drastic loss of quality when signal drops more and suddenly voice is totally unreadable under noise.