Yeah, it's not a night and day difference. Cruising on flat ground I noticed a roughly 2km/hr difference between my gravel and road bike at the same power, which is noticeable, and which is almost certainly due to a difference between wider gravel tires and shallow wheels and road tires with deep wheels (the aero tubing on the road bike probably contributes slightly, too). Trying out the difference on a climb there really wasn't really much to it, surprisingly - a couple of seconds over a 6 minute climb at the same power, which is basically down to the weight difference. Set up the gravel bike to put me in a similar position as the road bike, the bars are about 12mm higher and 2-3mm shorter, same width (36cm... I know), though, so they're directly comparable.
If I carry a spare aero wheelset with road tires, it's almost as good as having another road bike. That's exactly my plan on vacations, only space for 4 bikes on the rack, so I can't carry more than one, but I can always cram a spare wheelset in the van.
It's nice to have both, of course, but yeah, I'm totally digging the gravel bike.