Wider tires don't always necessarily mean slower. They will, however, mean greater comfort when going over bumpy/uneven/cracked streets. And fewer burst tubes. That's why all the FGFS kids try to get the fattest tires possible on their bikes, even if it means crimping their chainstays. The wider your tires, the more it'll be like riding a super comfy mountain bike!

25/28c is usually a decent size for comfort. i'm sure the kilo can fit 28c tires with no issue.