That doesn't depend on the text-to-audio modulator, but on the audio-to-baseband / audio-to-RF modulator.
With AM, audio amplitude is proportional to received signal strength, with FM, the received signal strength has absolutely no effect on audio signal (and hence, you can't recover the received power at all; that info is simply lost on the way).
In any case, I think you're not measuring the right thing – if you're really concerned about signal quality, you'd rather go for a measure of SNR in the audio prior to text de-coding. Received power is a meaningless figure without knowing how your receiver reacts to that, how much of that power is noise or interference, and how sensitive your audio demodulator is to that, and what the effects of audio distortions on your text communication are.
So, I personally think you might want to ask yet another question on ham.stackexchange.com: Given that I want to achieve X, how can I choose a digital/text data codec? and describe what that X and its environment is.