Total Starters Bench
NYK 18.0 13.7 4.3
OKC 13.6 7.4 6.3
SAS 10.1 8.5 1.7
CLE -1.1 -0.2 -0.9
BOS 2.9 -2.0 4.9
Above is the +/- for each indicated team, in the playoffs, broken down between starters and bench. By this measure, OKC has the best bench. I am surprised actually that NYK is as close as they are.
BOS has the 4th best +/- in the playoffs in their 7 games. I would say that BOS should have beat PHI, probably could beat CLE in a series (assuming they play to their potential), but are probably not there with OKC, SAS, and NYK. These +/- stats seem to agree.