Changing door speakers really doesn't need to be done at the same time as the HU. Though if you are paying someone to do the job I guess there may be a small cost saving.
I never found the 156 speakers a problem (5 years since I owned one though) but I am not a carpark blaster. Personally, when driving I prefer to hear the engine than the music. Music, for me, is background.
I say fit the HU and see what you think. It will all depend on personal preference anyway. What others hate you might like and vice-versa. Plus having others post that they are rubbish will persuade your brain that they are rubbish.