Electrical – What are the effects of using all of the 80% of the power supported by 15A circuit breaker (220 volts, #14AWG wire)

Most likely.

As long as the fixture has a UL (or equivalent) stamp/sticker, there is no problem.

Fixture wires, and the wiring inside devices is specifically designed to carry the rated loads. There are different rules applied in this situation, since the load is known and static.

According to the National Electrical Code 2014 402.5, the allowable ampacity of 18 AWG fixture wire is 6 amperes. Your fixture will only draw 2.5 amperes.

I = P/E = 300 Watts / 120 Volts = 2.5 Amperes

If the manufacturer could use smaller wires, they probably would. However, NEC says 18 AWG is the smallest allowed for fixture wires.

One of the reasons light socket adapters can be dangerous, is that they can allow more current to be drawn than was intended by the fixture manufacturer.

This ceases to be a factor if you use dedicated LED luminaires which don't have sockets. LEDs are so reliable, and they don't "burn out" like bulbs, that there is no reason to socket them and a lot of reasons not to - such as this.

This also opens the door to using more robust dimming controls like 0-10V, PWM or smart dimming. (The system you are currently angling for is "the old way of dimming" that is made for incandescents, and LEDs struggle to work properly because it works the exact opposite way LEDs do.)

If you really want to use those fixtures, you can switch their sockets to something like GU24 which do not support incandescents. However GU24 screw-in LEDs are notably more expensive. Candelabra sockets are readily available in LED at no cost disadvantage, but incandescent candelabra bulbs are commonly 40W and can be found as large as 60W.

If you really, really, really want to support incandescents, then use 2 dimmers and call it a "feature". However, you have other problems. This with the dimmers consumes more than 50% of a 20A circuit's capacity, so it must be on a dedicated circuit that serves only hardwired loads and does not power any receptacles. Like any continuous load, it must be derated 125% so this counts as 1500W toward hard circuit limits of 1800W (15A) or 2400W (20A).