The spectral 'purity' of a signal has little to do with SWR. It certainly can have a lot to do with the apparent power being transmitted and the quality of the transmitted signal. Keep in mind just exactly what SWR is. It's a ratio between forward and reverse power (actually measured by voltages but in this sort of thing they amount to the same thing). That means that depending on how bad the impedance mismatch is, so much of the signal, no matter what that signal consists of, is 'reflected' back down the feed line. That antenna or SWR meter doesn't know a 'good' signal from a 'bad' signal, both are treated the same. You should think of that signal as a 'whole', not what it's made up from.
Modifying a transmitter, in most cases, doesn't produce enough additional power output to make any significant difference in the strength of a signal. All those 'parts'/circuits in that transmitter's 'string' really are there for a legitimate purpose, not just to reduce power output. They do reduce power output of the 'wrong' kind, which you don't want to start with. In some very rare instances, usually where those additional parts aren't really needed, they aren't doing anything constructive, preventing something getting 'out' that shouldn't be getting out. But that's very rare. (The manufacturer isn't going to waste money on something that doesn't have to be there for some reason.) It isn't a matter of it being 'ham' or 'CB'er' radio, it's a matter of the signal being 'good' or not. It applies to any service, not just CB or ham stuff. Cell phones are radios, it applies to them too.
SWR only applies to impedance matching, it provides no useful information about anything else. It can't tell you anything about signal 'quality', if it has unwanted frequency characteristics or 'purity', or anything else. Unfortunately, it doesn't tell you all of what you need to know about that impedance matching to be very useful either. That SWR meter can be 'fooled' easily. To really understand that you have to know just what impedance is, and that is NOT something simple.
- 'Doc