This is interesting, I suck at Windows stuff so how do you go about changing the shell. If that's possible to make Steam boot.
You just go into the registry and change the shell from explorer to steam. You can find instructions anywhere online. Just type how to change explorer shell in windows 8 or whichever Windows you have. I forget what the exact registry string is you have to find but it's really simple.
Now the services are just as easy but on a decent rig you likely won't notice much, you type services.msc under MSconfig or find it in the administrator tools or search. Just to got blackviper.com and check out stuff that you could remove.....I just do it so I don't have interruptions or crap going on in the background that I don't use or need but it should help at tad as far as loading steam right away.
The big key is changing explorer, it really does boot up directly into Steam tons faster. It literally looks exactly like you booted into SteamOS and probably faster actually. Of course you change Steam settings to boot into big screen mode first.
When you want the desktop back for something like mods or whatever. Just hit CNTRL/ALT/Delete and go under File and select to run a new process. Just type Explorer and it starts running again.