Speaking from a purely theoretical standpoint rather than experience, I don't think it will have any noticeable effect. The speed of alt-tabbing from a full screen application has a lot to do with graphics, and the Steam overlay's graphics are approximately nil.
http://www.fimfiction.net/story/36641/My-Little-Exalt
IMO, the only good thing about having Steam and CoH running at the same time is so that you can use the voice chat interface with your friends who are also on Steam... now if we could use Steam's content delivery for updates that'd be great 
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I got a question about play CoH with the Steam overlay active. Does it affect how well the game reacts to Alt-Tabbing? In CoH, when I alt tab it's always very quick to switch between Firefox and CoH with Alt-Tabbing, but when I do that with my Valve games on Steam, it takes forever for the game to come back up when I switch back. Is that just because of the Source engine or does the Steam overlay aggravate the condition? If I add CoH to Steam so I can use the Steam overlay, will it make Alt-Tabbing in CoH unfeasable?
Issue 16 made me feel like this.
Warning: This poster likes to play Devil's Advocate.