One thing to remember with the numbers is that the chip may have some kind of built-in Tx power management by monitoring the actual power pushed to the air vs. power reflected back from the antenna. As the antenna itself seems to be a just a metal sliver with carefully formed shape suited for antenna use and the contact is a standard metal-gold-spring-loaded touch-gold-nickel-copper contact, it is subject to variance in quality from device to device and also WILL vary as the device ages and gets dropped, dusted, humid, squeezed, dipped in beer and so on. The Tx power can quite likely be controlled to some degree by the chip itself, though the problem here is not frying the gain stage - even the maximum output is but a candle compared to the supernova-like relative luminosity of the "real" transmitter stations - but power usage that should always be minimized. One nasty thing I noticed about the transmitter is that there's quite little fitering from the main power net of the phone to the transmitter. I noticed this when I was trying it with the not-that-good-but-adequate minihifi set I use as computer speakers and not that low sound level as I wanted to hear if the static changed as I played with the settings - the bundled charger distorted the transmitted signal to a horrible shriek that made my cat go from sleepy rest to VERY puffy in about 3,8 µs and resulted in eight bleeding deep puncture wounds on my shoulder, lots of swearing and bodily pain due to the shock from the shriek and the cat wanting to suddenly delocalize itself, which in turn resulted my computer chair toppling over and me lying on the floor, bleeding and wondering what the hell had just happened - and the shrieking just wouldn't stop until got up from the floor and pulled the charger cable - it was closer than the power button for the speaker set. Perhaps this is why the SW is not sure whether to allow the FM transmitter to work while a power-carrying cable is plugged in?
I've ran mine for a couple of hours at 118 and noticed no issues, but am generally using 112dBuV. IIRC an increase of 6dB will double the voltage.