Actually you can raise max tx power up to 120 dBμV according to "SI47XX PROGRAMMING GUIDE/AN332". The note is "Power may be set as high as 120 dBμV;however, voltage accuracy is not guaranteed", but who cares is it .975 V or 1.025 V

Further if you look into kernel code, in si4713.h there is a line

#define SI4713_MAX_POWER 120

so it looks like we do not have driver limiting max power.

And we should have in mind that difference between 118 and 120 is not linear as we are talking about dBμV units:

112 dBμV = 400.0 mV
118 dBμV = 794.3 mV
120 dBμV = 1 V

It seems that setting tx power to 120 works for me, but this could be placebo effect, so one can try and modify /sbin/fm-boost changing echoed value from 118 to 120 and check if there is an audible difference