I'll try to keep this relatively short.
Today, the M and R are partially constrained by terminal capacity at Forest Hills, ~20 TPH if I'm not mistaken.
SAS2 will require reconfiguration of Broadway service patterns to increase capacity to 125 St, ergo to potentially eliminate the Herald Sq merge and reroute the N via 2 Av.
Broadway CBTC will enable 30 TPH via City Hall, and thus via 60 St to Queens. I'll assume that would result in a roughly 2:1 split between Astoria and QBL, so 20 and 10 TPH respectively. That's, at the very least, a 2 TPH advantage to the R over today's schedule.
If Forest Hills is still the only QBL Local terminal, then any additional capacity on Broadway would have to be diverted to Astoria, and that minimum 2 TPH advantage would be lost.
However, with Queenslink in the picture, then the R would be the sole tenant at Forest Hills. The bottleneck would shift westward to the merge points with the M, while Forest Hills would have extra unused capacity. Therefore, R train frequencies could be increased regardless of terminal load.
To my understanding, since the R would be partially deinterlined in this scenario, then a case might be made to extend the R to Far Rockaway instead of the M, to take advantage of higher frequencies and 10-car consists.
Therefore, If we want to unlock the most capacity from SAS2 and Broadway CBTC, then we should also build Queenslink.