Going through and listening to live versions by the original band, it can be surprising how often they played some songs a half step down. There's a world of difference between hitting the notes in the studio after a week of rest, and hitting them 2 hours into the set on the 3rd month of a tour. Or in your case, the 40th song of the night on the second gig of the weekend. There's no shame in it. I think you should judge on a song by song basis whether or not it sounds too noticeable. If it allows you to expand your setlist, go for it. If it allows you to finish the night as strongly as you started it by avoiding serious vocal fatigue, go for it. With Vertigo, so far we've only dropped one song because a few of the notes were hit or miss for me, and we discovered that U2 played it down a half step live themselves.
I don't think audiences generally notice. I've always played 'She Talks To Angels' in open G, because I hate tuning to Open E. It's a pretty significant key change, although I actually end up singing it higher than the original, but no one has ever noticed.
By the way, if there is an easy song that not enough cover bands are playing around here (at least that I've heard), it's that one. It'll take 10 minutes for the band to learn, but audiences eat it up like candy.
"You can't just drop a measure. You'd end up...dropping a measure!" -Mojo