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Normally on a small L3 switches, QoS features are supported by the hardware. When they are, figure capacity as you might if QoS wasn't being used on the L3 switch.
In cases where traffic on the L3 switch is punted to the main processor, you have a problem. Often the main processor on a L3 switch has less, perhaps much less, raw CPU performance vs. a software based router. (This because, also normally, the main CPU only needs to perform control plane functions, not data plane functions.) Generally, vendors don't even provide performance specs for a L3 switch's CPU performance.
