It is not a silly question. "How far this sector antenna can reach" is a silly question The link budget formula is pretty clear so answer is Yes, mathematically Transmitter output power (dBm) and Transmitter antenna gain (dBi) sum. So you can alternate decrease in one with increase in another. This is to answer the question.
What needs to be mentioned: dBm and dBi come with the cost and there is no easy solution that covers every scenario. So the answer is also: No
- sector antenna with higher gain requires less radio amplifier output power which may help with signal modulation.
- sector antenna with lower gain requires more radio amplifier output power, which may be a challenge for higher modulation requirements not being possible at higher dBm power (depends on radio)
- sector antenna with higher gain collects signal better (antenna provides more amplification), but noise/interference gets more amplification with that antenna gain too. So in high noise environment, higher gain wide beam width sector antenna may not be always as effective as you would expect, because it collects the garbage signal from more distance and wider area. The more you can focus the beam the better in terms of noise immunity. We advocate for using only the sector antenna with gain that is just enough for the job, this is especially important on wide beam width sector antennas.
- higher gain sector antennas are usually bigger, heavier and more expensive, lower gain sector antennas are opposite.
etc etc, also consider things work vice versa and there are practical limitations to many. Considering the way how your question is asked we assume you have a good knowledge and are aware of the the complexity.
To decide on which type of antenna radiation pattern use, it is a question of customer density, distances to AP, radio platform, throughput requirements etc. If you need our assistance or opinion, please message @Tasos here on the forum or on Facebook / email him email@example.com according your preference