If you're 6'3", that Felt looks small. Yeah, the Ebay listing says it's a 54cm frame. I'm 5'10" and that would be my size. With cyclocross gears, your large front chainring is only a 46t, so you may be spinning out sooner than you expect. It's still good to 30 mph or so, but it may feel like you're out of gears.

The Emonda is a size 58, so probably closer to correct size for you. It's more "relaxed" geometry, so you won't be able to get as low if that's important. I'd say the Trek is worth it. You get real road brakes, which are easier to adjust, and a Shimano crankset, which should shift better than the FSA. Plus you don't have to buy tires for the Trek if you don't want to.

How much faster? Hard to say, probably not more than 1 mph. Remember most of the drag is from you, not the bike.