I think the raw thoughput numbers are 87mbps and 112mpbs for those routers.
These were tested using ethernet to ethernet with 64 byte packets running nothing else on the router.
To a point it will depend on what you are using to match your packets for dscp. Something simple like IP address won't hurt as bad as if you attempt to use some of the NBAR things you may have a issue.
This is partially the reason many features are not supported on a layer 3 switch but are on a routers. The switch wants to alway run at wire speed but the router is expect to run slower.
If you can get it to work a 3560 may be a better option that either the 2821 or 2851. In theory the switch should never be the bottleneck even at a full gig.
The issue will be does the switch contain all the features you need. If not you then must look at how much load that feature will place on the router.
If you really need a router at 75m and do QoS you may even need to look at a 3825 or larger.
