Maybe I'm the only one, but AvastSvc.exe uses more memory since the last update on my PC. Anyone else noticed it? If not, then it has to be something else on my machine causing this.
There will be no steady state because of the nature of an AV depending on what you are doing, will force one or more of the on-access shields to be running and that is going to generate RAM use. Since there was no avastSvc.exe in avast 4.8 but ashServ.exe, ashWebSv.exe and ashMaiSv.exe, there is no direct comparison (you would have to total all of those as a very basic rule of thumb).
Be assured that avast5's use of resources is lighter than avast4 for the same activity, the problem being is that there is no way to do a direct comparison unless you had two identical systems doing exactly the same thing, one with 4.8 the other with 5.0.
Honestly it isn't worth the time and effort worrying about it, the AV is there to protect you and it has to use system resources to do that. I guess you could stop browsing, running other processes, etc. and then avast would be using minimal resources only that to actually load the program (not scan).