PSK Bandwidth for SNR
I'm using a cosine-roll-off filter with a roll-off-factor of between 0.2 to 0.5.


Shamelessly copying from wikipedia's Raised-Cosine page, here's a plot of frequency responses of Raised-Cosines filters with different roll-offs:



As you can see, the cutoff frequency for all of these is always half the symbol rate, $\frac12 f_S = \frac12 \frac1{T}$, but the amount of energy outside of that depends on the roll-off. The closer you get to the perfect sinc shape in time ($\beta\rightarrow0$), the sharper that gets.

Notice that this leads to a (double-sided) bandwidth between the cutoff frequencies of $B=2\frac12f_S=f_S$!

However, even for the extreme case of a roll-off factor (which, by the way, is exactly defined to be the difference in occupied bandwidth compared to the minimal bandwidth given by above cutoff frequency) of $\beta=1$, you'd only get $B=2f_S$, so you actually consider the worst case for raised cosine filters – which is a justifiable simplification.

Notice that I'm presuming the book you're citing has a bandwidth definition that covers only a single side!