Telling you have 7 APs with no more information is useless, as it all depend on the size of the office and the existing obstructions. As previously told here, you need to conduct a Validation survey in your current deployment, but a good thing would have been to perform a virtual survey to see if the numbers match and then perform the validation survey.
With the previous been done, using 14 dBm for a 5 GHz radio is pretty fine and almost enough, but remember that maximum power transmission must adapt to the maximum power of the less capable device. If you are only using laptops, then setting Max Tx Power to 14, or even 17 dBm is fine, but if this is a mixed environment with smartphones, tablets and other stuff, then you could end up with power impairment. In this case, if their maximum Tx power is lower than 14 dBm the devices could be trying to transmit and this is not progressing.
Last but not least, as @Flavio Miranda said before, the client side is also important, as with the rapid evolution that we are having on the Wi-Fi nowadays, vendors don't have time to fully test not only the chipsets but also the drivers, and I have dealed with signal issues using Mediatek and Realtek chipsets, and they were fixed after a driver update.
