I find I'm normally quicker than my local F18's downwind especially in lighter winds and normally up to around 15mph.
If I'm defending my position I tend to sail lower than them but maintain the same speed as them and if I'm attacking I sail at the same angle as them and due to the differing weights go faster.
If the wind is so light I can't lift a hull then I let the traveller down the track up to 300mm from the centre. Ease the mainsheet just enough to open the leech and steer 50%to the spi luff and 50% for optimum VMG.
A little tip I've used in the past especially for longer courses is to centre the rudders and sit on the tiller, play the spi sheet to keep the luff tell tales flying and in my case sailing solo you find yourself doing very long 'S' curves down the course. Don't attempt this if you think there is enough wind to fly a hull! It's better to steer and sheet to keep the hull flying all the time and at all cost.
Going back to your question I don't think F18's have an advantage, both you and kathleen on your Blade are only a couple kgs more than F18 with no one on it, yea they have more sail area but they need it just to move. Sometimes this is an issue as once they are moving in the lighter winds they certainly don't stop as quick due to their inertia, where as us light boats tend to both accelerate and slow down quicker.
Maybe you just need a more positive attitude when competing against F18's in those types of conditions, once you start to overtake one or two or prevent them from overtaking you I think you'll find everything will change.