bro, if you are cycling your tank, you dont have to turn to 27. As mentioned by bro Piero, 28 degree is still possible. If you are keeping fishes and non demanding corals, you may go up to 29. Anyway, this is personal experience. Hence, some ppl opinion may differs. It is really up to you.
As mentioned, your unit is facing west sun, hence i think your water will heat up faster. However, the chiller will just kicks in more often during that period. Maybe it is a good time to test whether your chiller can really support your tank by setting it to say 26 and see how long it takes to cool down and how fast it takes for the chiller to kick in again.