The benefit of segregating user groups by line-of-business (using logon groups) is related to the point that groups of users (like SD users or HR users, for example) tend to use the same sets of data. They (generally) work with the same groups of tables and hit the same indexes using the same programs (transactions).
So, if you can group all of the users hitting the same tables, onto (or one set of) App server(s), then you can tune the App server buffers to a much greater extent. If the FI users (generally) never hit against the HR tables then the App servers in the FI group don't (generally) have to buffer any HR data. That leaves you free to make memory and buffer adjustments to a more drastic extent, because you don't have to worry (as much) about screwing the HR users (as an example), when you're adjusting the FI server group.
So, (in opinion only) you should start with a buffer hit ratio analysis / DB table & index access analysis (by user group) to see where you would get the best benefit from this kind of setup. If you don't have this kind of info, then creating logon groups by line-of-business may have no benefit (or worst case, may make performance degrade for the group with the highest load %). You need some historical information to base your decision on, for how to best split the users up.
You may find that 50% of the load is from the SD users and so you may need one group for them (with 3 App servers in it) and one other group for everyone else (with the other 3).
The logon group(s) will have to be referenced by SAP GUI, so SAP GUI (or saplogon.ini + maybe the services file, only) will have to change to accomodate any new groups you create in SMLG. Also consider that there's variables for time-of-day (load varies by time-of-day) and op-mode switches (resources vary by op-mode).
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