The data rate is still Rb. The functional "p" seems to be a spectral operator, do clarify on exactly what you mean by "p".
If you want to know the implication of the roll-off factor on the bandwidth, then that should not be a concern.
The roll off is there to create a more realizable filter. Ideally $\alpha = 0$.
ISI:
If $\alpha=1$ then you have a signal with an ISI assuming you have the same sampling rate of 2W, wherein the op= current_ip + past_ip.
Typically systems are causal so you consider the impact of the previous ip not the future ip.