You have to be a little cautious with GPS speeds. The figure tells you how fast the GPS unit's aerial is moving through space at a specific point in time, it isn't directly measuring your boat speed as such. If you are swooping over the crest of a wave, the unit itself is being sung rapidly through an arc, and this can result in a momentary speed peak which is way out of the ordinary. Similarly when you gouge through a trough in the waves, the boat as a whole is clearly still moving forward but your GPS speed might apparently drop right down to 0. The temptation is to see the maximum speed recorded and get the thrill that you are really blasting along at that. I made a mad dash to the Solent from Poole in the summer in a following F7-8 wind and apparently hit 9.1knots at some point (I didn't see it, I had other things to concentrate on, like surviving) I don't think I was doing that really, the unit was reading around 6-6.5kts much of the time, and the moving average was a more modest but very creditable 4.5kts.
Having said that, BayRaiders and Cruisers can clearly plane, especially unballasted. They are just big dinghy's with flat bottoms after all. I should say anytime you are holding a speed of over 6kts you are planing. If the wind increases, the sea is smooth and your nerve holds, the sky is the limit after that...
By the Way, my spell checker just suggested the correct spelling fro BayRaider was backslider...