I like Luke better than Lucas (which inevitably reminds me of mucus!), but I should think they'd get over it. Luc is used on it's own in France, and Luke is the form used in the Bible (although it comes from the Greek name Λουκας (Loukas)). I think there's alternatives, like Lucian or Lucius or Luka (I like Luka more than Lucas, but I'm still not the hugest fan).
I know a kid named Jon Luke, who has almost always gone by Luke, and he's never seemed to have any troubles with it (although, as a kid, I remember giving his mom flack for not spelling John "properly", haha! Once a name nerd always a name nerd, I suppose...). Although I remember a member on here who has a just Luke, and at his school one of the administrative ladies was insisting that he had to have a full name and that Luke's mom was lying and to just tell her his name was Lucas already. :/ Some people are just losers, hahaha! I think everybody will face name grief like that every now and then, but if you love the name, I would go for it. It's quite handsome.
I wouldn't assume Lucas either. I might ask if it was short for anything, but only out of curiosity - not because I assumed it was.