No. The Stereo Mixer does not use anymore resources in 5.1 than 5.0. In 5.1 (just as with 5.0), you can get 59 inputs per stereo mixer. If more resources were used then it would be impossible to get the same number of tracks.The only thing that might cause confusion is that in 5.0, tracks with offline voices don't use mixer resources, where as in 5.1, they do. This was changed in 5.1 due to the added the ability to make tracks inactive (which is a "better" way to reclaim resources that aren't needed).